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What is Cob?
Cob is earth used as a building material. Straw is mixed in with it to improve its strength.
If your soil is too sandy, you add a little clay. If your soil has too much clay in it, you add a little sand. If you are lucky, your earth may be just right. We call this ‘ready-mix’.
You don’t use any old soil. You discard the top soil, the stuff that gardeners love. It is full of decomposed leaves and micro-organisms. You are after the soil below that, the sub-soil, the inorganic material. Nobody wants that. But it is exactly what you need. One man’s junk is another man’s treasure.
You need to dig up the earth to lay your foundations. Instead of dumping it, you will turn that earth into the building that will sit on top of those foundations. That’s pretty amazing.
You have your supply of sub-soil with the right balance of sand and clay. Now it is time to mix in your straw. You can do this by dancing on top of it, by letting livestock trample on it, or by mixing it using various basic machines. Using livestock to trample the cob overnight is a very ancient (and sensible) method. Animal dung can improve cob’s workability, but be careful of this approach if you live in a country with dung beetles; I saw one destroy a mud-dung floor in Oregon. It looked like a tiny mole had crisscrossed over the entire surface (being cob, though, it was very easily repaired).
From an environmentally-friendly point of view, it does not get much better. You source your raw material yourself, right where you want to build. No tree is cut down, no rock is quarried, no metal is mined, no oil is extracted.
Your raw material does not require any melting or heating at high temperatures, or the addition of any chemicals or massive quantities of water, to turn it into a building material.
There is no need for transport from the forest/quarry/mine/rig to the factory; no transport from the factory to the builders’ providers; no transport from the builders’ providers to your site.
Cob is not toxic. It will not harm your health if you live a lifetime within earthen walls.
In generations to come, if your home is no longer occupied, cob will eventually disintegrate back into the earth. It will leave no trace. That is a pretty special quality in a building material and often all too rare. When checking the 'green-ness' of a building material, you need to think about its whole life cycle - from the cradle-to-the-grave. If you would like to read a bit more about this, please click here.
Cob goes even one step further. It is perpetually re-usable. This is known as cradle-to-cradle. When a material has finshed its first tour of duty, it can be used a second time. In the case of cob, it can be used a third, fourth, fifth time. By adding enough water to a cob wall, you can actually re-sculpt it. If you knock a cob wall down (for an extension for example), you can simply add water and use that original cob to build your new wall.
I visited a 150 year old Irish cob cottage in 2009. The wall had been severely damaged due to the application of cement render in the 1950s. The owner had the foresight to gather up and store the crumbling cob. The wall could be rebuilt using the original material. Isn't that wonderful?
It is free, except for the labour you put in. You do put in a lot of labour.
Be aware too, that you will need to roof your cob home, have built-in furniture, etc. It is more time-consuming and trickier to add these elements to a curved building. If you need to pay trades people to help you with these aspects, it will be more expensive than if they were fitting out a rectilinear house. There are tricks that we can teach you to minimise this kind of expensive input. Check out our courses.
At the end of the day, your cob home should not cost any more than a standard-built house. It is possible to build it cheaper than a standard house. We certainly did. At the end though, you will not have a standard house. You will have created something beautiful, sculptural, personal and enduring.
Anyone can build with cob, small children and grandparents alike. Personally, I am nervous of power tools. There is no need for them when cobbing. When a batch of cob is made, traditionally it is rolled into ‘loaves’. You don’t need strength to carry it from A to B. You size the loaf, or cob, to your ability. Each loaf gets worked into the monolith of cob below.
If working on a larger scale, you can pitchfork or shovel cob into place and work it in with simple wooden tools. Whether working with small cobs, or larger shovelfuls, this is building at a human scale (something which is absent from so many contemporary buildings, with all of their components craned into position).
Cob is sculptural. You can curve it. You can carve into it. You can add on to it. It can be reworked at any stage in its life. Cob works really well in curves. A curved cob wall is actually stronger than a straight one, as it becomes self-buttressing; it supports itself. This opens up so many possibilities for a completely individual building, full of personality and free from the 1.2m x 2.4 (4ft. x 8ft.) module dictated by so many modern building materials.
Curved or non-uniform rooms feel good. There is a theory that as humankind evolved in nature, we can only be truly comfortable when we are in ‘natural’ environments. The box-shaped rooms that most people live in nowadays are, in fact, alien to us. We are not meant to spend so much time around so many straight lines. Modular, straight building components are the result of mechanisation for mass production. They are driven by profit margins and convenience, not by any regard for the health or well-being of the future occupants of these buildings. A curved or randomly shaped room feels like it is embracing you. There are no dead corners. There is a flow. It is a pleasure to spend time there.
Architecture should be about all the senses. Too often, architecture is purely visual. Cob buildings are extremly tactile. The first thing that people do when they visit our house is carress it. They touch the walls, they follow the line of the curves with their hands. Visitors are surprised at how much they want to do it, but everbody does it, spontaneously and without exception.
The mass of earth and non-uniform shapes of cob also allow for wonderful, gentle acoustics - that's the sense of hearing covered. Cob buildings do not really smell - perhaps the slightest hint of earth - an outdoors, natural smell. There is a definite absence of chemical smells. Cob buildings look good enough to eat. Geophagy, or 'eating clay/dirt/earth', happens worldwide for cultural, dietry and/or medicinal purposes. I wouldn't recommend it, but maybe cob can really claim to satisfy all of the senses.
Cob is labour intensive, but it is extremely satisfying. As there is a slow food movement, so cob belongs to the slow build movement. Cob-builders are not getting a consumer product instantly off the shelf. They are spending time, crafting their building, taking great pride in their work. The building process may take longer than usual, but the legacy will last for generations.
Cob buildings are built as monoliths – huge, thick, solid walls. Traditionally they were built approximately 600mm (2 ft.) wide, up to a storey-and-a-half or more.
They need ‘a good hat and good boots’, large overhangs and stone or block plinths to minimise the amount of rain reaching the cob walls. They need to be finished with compatible materials, which will protect the cob but also allow it to breathe.
If you maintain your cob building well, such as re-newing your external limewash every few years, it will last for generations.
Cob does not have a "good" u-value. The current building regulations in Ireland require that the maximum u-value for a wall should be 0.21 W/m²K. A 600mm wide cob wall will only achieve values from approxiamtely 0.4 to 0.65 W/m²K, pretty far off the mark. This implies that cob walls need to be heavily insulated to achieve current standards of thermal comfort, but our experience begs to differ.
The building regulations do not take into account cob’s excellent thermal mass properties (the walls act like slow release storage heaters), its monolithic nature (which has a effect on how heat passes through the wall), its thermal inertia properties and the fact that the mass of earth in the walls is a huge reservoir for water vapour (reducing humidity levels in a room, which allows you to feel more comfortable at lower temperatures). These qualities need to be investigated and measured further if cob is to be accepted as a viable modern building material.
In our house, we compensated for cob’s alleged inadequate insulation capability by super-insulating everything else – the floor, the roof and the two coldest walls of our home (north and east – these are timber-frame, with straw-bales for insulation and cobbed up the inside with 100mm (4”) of cob). In the winter, our house is toasty with only a 10kW turf-and-wood-stove to heat 130m² of floor area, as well as our hot water. We have no back-up heating system (we have solar water panels for our summer hot water). I cannot feel any difference between the rooms which are entirely surrounded by cob compared with the rooms which are partially or totally super-insulated.
The problem of straight-forward compliance with the building regulations will be an issue for anyone wanting to take on a cob building. If you want to read more about this, please click here.