Glorious Mud
In the course of what passes for an education in your country you may have been told that poor people in the Olden Days often lived in homes made of wattle and daub, that is, a framework of sticks with mud slapped on. You may also have heard tell of people in a place called 'Africa' who live in mud huts. If so, it is almost certain that you were given the impression that these are inferior dwellings: after all, mud is dirt, and dirt is dirty, and certainly not as sturdy as proper construction. Depending where you live, ideas of what 'proper construction' is may diverge: in Germany, rendered walls made of concrete blocks are the way to go, in England it has to be brick, while in Scandinavia and North America they prefer wooden frame construction, presumably because it helps get rid of the excess trees.
But the low status accorded to mud in construction has more to do with social history than its intrinsic qualities as a building material. Mud is plentiful, cheap, and easy to process, and historically was therefore likely to be used by poor people. Stone construction was more difficult and expensive, and therefore only available to the rich, who chose it for their homes to demonstrate their status, and for their religious buildings to demonstrate their devotion. And their status. If you free yourself from prejudice, it turns out that earth/clay/mud/whatever you want to call it has only two drawbacks as a construction material: it doesn't respond well to being rained on, and it is not quite as strong as stone. The first drawback can be overcome with simple constructive measures and proper maintenance, of which more later, the second is only relevant if you want to build something very very high.
The Options of Mud
When used in building, mud has to be properly compacted into a solid homogeneous state. You can do this in situ as you build, or beforehand, and you can do it with or without a mould. Combining those two choices in theory provides you with four basic categories of earthen construction, but real life of course is different:
If you prepare your clay beforehand by pressing it into a mould, and then build with the resulting unfired bricks then the technique you are using is known as adobe. Entire towns in the Americas and in the Middle East are built of adobe, as are several interior walls in my house. In England the technique was called clay lump building, and was used from the late 18th to 19th Centuries years in parts of East Anglia.
If you squash your material into a mould in the shape of the wall you want to build and in the place you want the wall to be, you are engaging in one kind of rammed earth construction. The Romans are known to have made rammed earth walls this way, but they weren't the only ones. Sometimes earthen infilling in half-timbered buildings was made of rammed earth. If you make a mould for the entire wall and fill it in layers that you compact one after the other, you are working in pisé. In the rare technique of shuttered earth, a smaller mould is used, which is moved up the wall as it is build.
For the sake of mathematical completeness I will mention the option of compacting your mud off-site with no mould and building using the resulting amorphous mud blobs. As far as I know, no culture has developed this option.
The infilling in half-timbered houses was more often made of wattle and daub, which is easier to make in irregular shapes, since no mould is required: a light framework of laths canes or sticks is made, onto which a thick layer of mud is daubed from both sides. The main strength of the wall is provided by the dried clay, but the wattle, apart from providing support during construction, also reinforces the finished wall to some extent, since it is less prone to cracking than the clay, holds the clay sticking to it together, and stops any cracks from propagating through the wall. A similar effect is provided by mixing straw, hair, or even wood chips into the clay mix, and most clay construction is done with such a mixture. (You may recall how upset the Israelites were when Pharaoh told them to make adobe bricks without straw.)
If you decide to compact your earth on the spot without a mould, wattle and daub is not the only option available to you. You can also build with rammed earth just by putting the earth where you want it and hitting it to compact it. If you read about 'dirt' floors in primitive or poor buildings, it is likely that they are in fact rammed earth floors made this way. Many roads are still built with this technique, as were the original fortifications of the Peter and Paul Fortress, in St. Petersburg. But if you build a wall like this, then what you are doing is 'cob construction.'
Traditional Cob Construction
Cob construction has been used at many times and in many places, but the area with which it is most associated in the English-speaking world is South-West England and South Wales, which are characterised by the widespread availability of suitable sandy clay subsoil for construction and the relative scarcity of the wood that would be required for most alternative kinds of clay construction. Cob buildings were being constructed there by the 13th Century at the latest, and continued to be built until the early 20th Century. The use of cob declined after the early 19th Century with the availability of cheap industrially-fired bricks. In recent years there has been a modest-scale revival in cob construction, in parallel with a generally greater interest in pre-industrial and craft techniques.
The success of the technique in what is by no means the driest part of the world demonstrates that the sensitivity of clay construction to rain is far from being an insurmountable obstacle. Wetness in and of itself is in any case no problem: what is needed is protection from flowing or standing water from above or from below. This was traditionally provided by supplying cob buildings with 'boots and a hat': the bottom of the wall was made of stone, and the roof was made with wide eaves to make sure any drips or rivulets coming off it were kept safely away from the wall.
Cob construction therefore started with stone foundations up to a foot or two above ground level. Gobbets of wet subsoil ('cobs') were placed onto this and then trampled or clubbed into a solid mass. After about half a yard (50cm) had been added to the height of the wall, the top of the wall was covered to keep the rain off and it was left to dry for a week or two. Then the messy bits on the inside and outside were trimmed off and the next 'lift' was applied in the same way. The process was repeated until the wall reached the desired height. Doors and windows could be provided either by building an entire frame into the wall as it was built, or by embedding the lintel in the wall and then cutting out the wall from underneath it later. To finish off, the roof was added, and the walls were whitewashed inside and out for added protection. Dwellings would usually also be plastered with a stucco of clay or of lime and sand. Sometimes the outside walls were only treated after the house had been in use for a while, to give the walls plenty of time to dry, but in any case a completely impermeable rendering would have been inappropriate: any moisture that gets into a clay wall tends to spread throughout it, and it should therefore be able to get out of the wall again wherever it ends up. Clay walls need to be able to breathe.
As in earthen construction in general, straw was usually added to the soil to reduce the likelihood of cracking, and sometimes flint or crushed shale was included in the mix. The resulting buildings lasted literally for centuries, and many are still in use today. The technique fell out of use because it was old-fashioned and primitive, not because of any intrinsic deficiencies. Mud was unfashionable in a world that wanted progress. Cob construction was used occasionally in restoration work between the Wars, but then fell into desuetude.
The Cob Revival
The first cob construction built after the War was a bus shelter built in 1980 in Down St. Mary, in Devon. The builder, Alfred Howard, had learned the basics of cob construction from his father while rebuilding and repairing old cob structures in the 1930s. In the course of his work as a builder after the war he and his workers worked on and in cob houses from time to time and he told them that one day they would build a complete cob building so they could see how it was done. One day he saw a bunch of children waiting for the bus, and had an idea for what building that would be.
Various factors came together to make the small structure that resulted one of the most influential bus shelters in building history. In Europe these days it is regarded as normal that historical buildings will be well-cared for and when necessary properly renovated in a way that respects the techniques originally used in their construction. For most of history, however, an old building that was still being used would normally from time to time be 'improved' using contemporary technology and in accordance with contemporary tastes. Over the centuries this led to the loss of much that we would now find beautiful, but until the 20th century it seldom led to serious structural damage. By the 80s it was clear that using modern materials and techniques to repair older buildings could often damage them. The damage that had already been done was in need of repair, and that repair would require a knowledge of the old materials and techniques.
Apart from that, as the economy grew, tourism grew more quickly, creating a demand for visible and visitable history and providing revenue that could be used to provide it. Many dilapidated historical buildings were repaired, and many that had been given over to mundane uses over the centuries were recovered for the cultural industry and de-modernised. This amplified the demand for historical construction know-how. And one of the places people went to acquire that know-how was the site of the Europe's newest cob building.
As knowledge of past building techniques spread, so did knowledge of their advantages. Earthen walls in particular were found to function as humidity regulators, absorbing moisture from the air when it was humid and releasing it when the air was dry. Apart from the effect this has on human comfort, it also reduces the amount of expansion and contraction of any wood used in construction. (This may be why my house creaks far less than you would expect of a wooden house in a variable climate.) And increasing knowledge of the advantages of earthen construction had the result you would expect: that people started using it in new construction and not just in restoration projects.
In 1994 Kevin McCabe built the first modern traditional cob house in East Devon. Since then he and others have built more, generally aiming at the higher end of the market, though this has more to do with the structure of the UK house and land markets than the cost of the construction itself: the cob outside walls for a £300,000 house cost all of £20,000.
Meanwhile, over in Oregon, a place some of you may have heard of, Ianto Evans and Linda Smiley had set up the Cob Cottage Company. Ianto Evans was from Wales, where he had seen cob houses, and wanted one of his own. Given the lack of any living tradition of cob building in the North-Western United States, the two of them developed 'Oregon Cob' in the mid-1980s on the basis of reading and experiment. After building a house that didn't fall down they set up their company and started building for others and teaching the technique. Rather than just digging up the local subsoil they used (and use) a standardised mixture containing more sand and straw and higher-quality clay than traditionally used in cob construction. The result is a stronger material, making it possible to construct thinner structural walls and have smaller-scale detail in the structure.
Pros and Cons of Cob
So, would you want a cob house? It all depends:
On the plus side, a cob wall is a lot of material. It takes a long time to warm up and a long time to cool down. So a cob house tends to stay cool in the summer and warm in the winter. The clay regulates the humidity in the house, making it more comfortable, reducing the likelihood of bad hair days, and protecting your respiratory passages. If you make your walls thick enough, they will take care of heat insulation on their own.
On the downside, a cob wall is a lot of material, and it takes up a lot of space. The walls will need to be about a metre thick if they are to provide adequate insulation. Cob walls are less tolerant of neglect than stone or brick: you will need to keep your roof in good repair and keep whitewashing. Large windows are difficult (although not impossible).
On the neither up nor downside: it is easy to make curved walls out of cob. Once you have made them it is as difficult or easy (difficult) to divide up the resulting space into rooms as it would be if you had made the walls out of anything else. Some Oregon Cob houses look like something out of a Disney film of a fake Tolkien novel. You may find this a good thing or a bad thing. But the aesthetics are not inherent to the material. Straight lines are just as easy.
Sources and references
http://www.ccathsu.com/alternativebuilding/cob/jeffreySP2005/index.html
http://www.networkearth.org/naturalbuilding/history.html
http://www.cobcottage.com/whatis
http://www.davidsheen.com/firstearth/interviews/howard.htm
http://www.buildingconservation.com/articles/earth/earth_buildings.htm
http://www.alternativesmagazine.com/17/kemery.html