Cob is a mixture of sub-soil which must contain some clay and straw (usually wheat or barley). The amount of clay will vary from site to site, depending on local sub-soil make up; it is sometimes worth adjusting the balance of aggregates to reduce shrinkage, increase weather resistance and compressive strength in the final mix. Approximately one third of the world’s population live in earth buildings of one sort or another.
Cob is unusual as an earth building technique in that it is a monolithic structure, which is built without shuttering, with the straw used to hold the mix together in its early wet state. In this way after a short drying period in can be pared down to the required shape and form. This is done either with a sharp spade or mattock, then the surface is dressed up with a heavy mallet and can even be finished with the bare hand. Thus creating a hand-sculptured building with wonderful possibilities for pleasing form.
Why Use Cob
As well as the important aesthetic of form, colour and texture and an automatic blending with the local environment (as long as your sub-soil is locally sourced). Cob has several other important characteristics, firstly walls are usually very thick, typically 2-3 feet and this, as well as being very good insulation against heat and cold is a massive heat store. So there is a fly-wheel effect on thermal movement – in other words, cool nights and warm days are evened out. When doors and windows are left open, resulting in a change and cooling of the air in a building, as soon as they are closed the building warms up again through the heat stored in the thermal mass of the walls. This means that typically a cob house will use approximately 20% less energy to heat compared with a typical modern house meeting the same building regulation insulation requirements. Another important factor, often not appreciated, is the humidity store of earth walls, they can easily absorb moisture from the atmosphere and release it again when the air dries out thus resulting in an overall higher background humidity than a typical new building. This is much more comfortable and healthier to live with, not drying out airways and resulting in less susceptibility to throat infections such as colds.
There has been much research to endorse what I’m saying here, particularly by “CRATerre-EAG” at the school of architecture of Grenoble in France, and Professor Germont Minke of Kassel university in Germany. Most of the new building projects in these countries have been done in either rammed earth (a form of building, not native to the UK, using a drier mix of earth and shuttering, resulting in a quicker drying time but much less sculptural characteristics), or in adobe (earth bricks). Minke has also experimented with “sausages” of an earth mix on the inside of a highly insulated wall simply to get the benefits of some thermal mass inside the insulation and mainly the humidity store. Personally, however, there is nothing more satisfying than the massive three-dimensionally sculptured forms of cob, as I hope you’ll agree when you look at some of the photos available on this site.
So what are the disadvantages of using cob?
Well, the fact that it is mixed wet, means in a thick wall it will take some months (depending on site conditions usually 6-9 months) after building, for the walls to finish shrinking. Most of the shrinkage will occur in the height of the wall, this means in practice waiting around a week of good weather between each lift (a lift being two feet or so in height) before it is sufficiently dry and strong to take the next lift. Therefore three months is ample time, allowing for a typical English summer to build a two-story building.
Although the roof structure can go on a week after the wall plate height is reached it’s best to wait about six months before fitting windows and door frames, otherwise there is a danger of the reveals either side of the opening shrinking and then the lintel crushing the window frame. This means it is usually fifteen months from starting to finishing a cob house.
So far, I have built over seventeen buildings of one sort or another and all of them, except two rather complex split-level part-retaining walled buildings, have been finished well within this time frame. Although this is longer than a typical new estate house, it’s not actually especially long for any bespoke one off design building. It’s also worth noting if any green oak is used in the structure for lintels or roof structure, then this will continue to shrink for much longer than the cob (about one year per inch thickness of timber).
Another disadvantage is that usually site space is at a premium these days and the simple fact that to pass the latest “part-L” insulation performance requirements for building regulations, the exterior walls will probably need to be three feet thick. This means the whole building will take up a significantly larger footprint and also need a larger roof area all around for the same internal space. However, as long as the site is not really tiny, this is a small price to pay.
Is cob more expensive?
No. The actual cob work with the aid of modern machinery, namely a JCB 3CX, is surprisingly cheap, it will depend on site accessibility and how much adjustment is required to the onsite sub-soil, but usually the cost of the cob itself, including labour and materials will be no more than £200 per cubic meter. In practice this means between £15,000 – £20,000 for a typical three bedroomed house. There are wider footings to consider and a stone plinth which brings the price up a bit and of course if you have many curves it makes most building processes (except the cob itself) more time consuming. In fact the walls of a new house are usually less than 25% of the cost of the whole building. In the case of cob, it’s usually about the same i.e. half of this 25% to put in footings DPC and build a well insulated stone-faced plinth to a height of around three feet and the cob work up to roof height about the same cost again.
The main criteria in the cost of a new-build project, apart from the initial site cost, is how high the finished spec is i.e. in the case of Keppel Gate we have bespoke sweet chestnut mullioned windows, oak floor boards, under floor heating, Aga cooker, hand made oak kitchen, marble bathrooms, porcelain floor tiles and hand made oak doors with hand made iron furniture. Therefore this is not a cheap build, nevertheless because of the efficient design using the wide roof space to create a three story building, the square meter build cost in 2001 was £800, which for such a high spec is very reasonable.
Today’s overall price per square meter for similar quality would be in the region of £1,500 to £2,000.