Resume Of A Cob Wall

[Charron]

I've compiled a pro-con description of a cob wall, as described by the authors of Building Green: A Complete How-To Guide to Alternative Building Methods, to help you determine if this building style is right for you.


Cob Wall

What is it?

"Cob" is created by mixing dirt, containing clay and aggregates (sand, stone, etc), with water to create a mouldable mud, and with straw or another fibrous material to assist as a binding agent. The proportions of each material used to make cob are variable and any mix should be thoroughly tested before being used. A series of testing methods are described in detail in the “Building Green” book mentioned above.

A cob wall is referred to as a ‘monolithic’ wall system because the cob comprises the entire wall system, including supporting structure, infill, and finish. Sometimes a decorative finish coat of plaster is applied to a cob wall, but it is not necessary. Cob style walls, under many names, have been used for centuries. The existence of ancient cob structures today attest to thier durability and practicality.

Advantages

- materials are readily available, inexpensive, and often found on-site. A simple test of your soil could be made with a bit of water and a mason jar.
- construction method is relatively simple and requires little skill. Ever made a mud pie? You're half way there.
- walls are very strong and durable, even in high rain environs. It's a commonly held belief that walls made out of dirt will just melt back to mud in the rain. The clay content makes the difference, and the walls stand up to more than you might think.
- walls are hygroscopic, absorbing and giving off moisture to regulate humidity. Regulated humidity discourages mould growth, improving indoor air quality.
- very accommodating for creative flexibility and intrinsic beauty. 'Organic' to 'Stoic' structures can be created using cob. It is very flexible, and using a technique called 'corbelling', it is vertually limitless in its structural possibilities.

Disadvantages

- cob building is HARD WORK. The sheer volume of material that is used to make a cob wall is daunting, but the processing of the materials into a useable cob, and installing it, is very physically demanding. Several mechanical methods have been experimented with to ease the workload, but in the end there is still a lot of heavy lifting and significant physical labour
- having more people is not always better. While work crews, or work ‘parties’, are a great idea, only one course of cob can be laid at a time. Each course must have time to cure, so if you have too many people for the length of your cob wall it may become frustrating.
- It takes longer than you think. Because each course needs time to cure, only a maximum of one course per day can be laid. If you believe you can lay a the full length of your wall each day, calculate the time it will take as: finished height of wall divided by 6 inches, multiplied by one day.

Appropriate Climates

There is a debate about the existence of ‘Mass Enhanced R-Value” of a wall system. It is commonly accepted that heat will move through things towards an area of less heat. Another common belief held is that light airy things are better insulators (preventors of this heat movement), and that heavy dense things are not. A cob wall is a very heavy, dense thing. But is it a good insulator?

R-Value is the standard measure of a material’s ability to resist ('R') heat flow through it. It is tested in laboratories by applying a constant temperature on one side of the material, then measuring how much energy needs to be expended to keep the other side of the material at a different constant temperature.

From the perspective of ‘thermal mass’ a cob wall is equivalent to earth, and so has a great capacity to absorb, and give off, heat. In a laboratory situation of consistent heat applied to one side it makes sense that the cob wall would constantly absorb the heat through its thickness. If the opposite side attempted to maintain a lower temperature it would become very difficult to maintain that constant different temperature.

However, most of us do not live in a laboratory setting. A situation where a consistent exterior temperature occurs is not common. This is where the theory of Mass Enhanced R-Value comes into play. It brings an added concept of “Time” into the mix.

The concept assumes that exterior temperatures will be variable throughout a given day, and that the thickness of the mass will be able to withstand the transfer of a temperature fully from the outside to the inside before the outside temperature changes. For instance, if daytime is hot in your area and your cob wall absorbs the heat gradually through its thickness throughout the day, but your night-times are very cool and allow the heat collected in the wall to travel back out to the night-time air, your cob walls would be considered as having effective Mass Enhanced R-Value.

If, like in most of Canada or, for example, northern Michigan, you have consistently cold winters (a.k.a. consistently cold outdoor air temperatures day and night) the capacity of a cob wall to act as a mass enhanced R-value barrier is vastly reduced. The constant cold outdoor temperature would promote the transfer of the higher indoor heat through its mass consistently, affording no insulating value whatsoever. This problem could be overcome only by consuming excessive amounts of indoor heating fuels, whatever they may be.

Ideally, a cob wall would be used by a person living in fairly temperate climate where the daytime and night time temperatures are fairly different and consistent. For those of us living in more variable climes a sun-facing direction, where solar gain can be a factor in providing heat to a cob wall, should be the only use.

[BambooNut]

Nice write up.

There was an article in the Lubbock Avalanche Journal, about someone building with cob. He is mostly building outbuilding structures, the example was a sauna.

The city building inspector was quoted in the paper as saying:

"We would be estremely careful in evaluating something like that."
AND!!
"Is this thing going to melt after three rains?"

I suppose he is not aware that cob is a common historical building material,
and that there are cob buildings in Europe which are HUNDREDS of years old, and still occupied.

This is why I am mostly looking at oultlying areas to place my home.
I can barely stand bureaucracy when everything is simple.
I do not relish having to educate the codes inforcement people every step of the way.


More importantly, I am going to work on getting in touch with the builder.
His name was in the article, with his E-mail, but the adress as printed, looked wrong.
I attempted to fire off an email and it bounced, as I thought it would.

I am a bit hesitant to call just out of the blue.

It is nice to see an article about someone actually doing something around here.

[Charron]

It would be very cool if you could get in touch with that guy. You could invite him here, to the forum, so we could all benefit from his experiences.

On the other hand, any of us could invite our code inspectors, or potential code inspectors, here. They could use this forum to let us know about particuar concerns in any given building style, then perhaps we could discuss it and come up with a mutally acceptable solution. Who knows, maybe eventually this could become a reference source for code inspectors to educate themselves enough to support the progress towards sustainable building.

For those of us surging ahead with projects, I say Go! We greatly appreciate your willingness to brave the potential frustration and headache when dealing with bureaucratic hoopla. Eventually the 'newness' of the 'old stuff' will wear off and we can go back to dealing with the normal, though still potentially frustrating, code approval process.