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Frequently Asked Questions about Cob Construction
Does cob work in humid climates?
Cob is a form of earthen building developed in the British Isles, a very wet area famous for beautiful mists and vicious storms. Yet the native cob buildings in these places have withstood centuries of harsh weather and still remain in use today. When I built a cob studio Pensacola, Florida in 2002 many people were skeptical that it would survive our 98% humidity and frequent hurricanes. However, the building has had no problems despite soaking rains throughout the building process and a hurricane that destroyed many of the surrounding wooden buildings. Even in humid climates cob does not rot, grow mold, get eaten by termites, or melt on the rain.
Cob is very porous and can absorb a tremendous amount of water without softening. In fact, unless it is completely submerged, cob will never just “melt”. It will however erode over time if it is exposed to direct rain. The solution to protecting cob from moisture is “a good hat and boots”. In other words, a cob house needs a good roof with wide eves and the bottom of a cob wall must sit on a non-absorbent stem wall so that it cannot wick moisture up from the ground or be splashed by water coming off of the roof. Cob walls are very thick, often two feet or more, so even the heaviest rain will never soak through more than the outer layers of plaster. Even if your walls get soaked with rain occasionally (like in a really big storm) they will be fine as long as they can dry back out. For this reason it is very import to never plaster a cob wall with a non-breathable coating like cement stucco.
Why can’t I just seal my walls with cement stucco?
Don’t make the mistake of trying to protect your cob walls by sealing them from moisture. Even the best stucco job will inevitably develop tiny cracks and allow moisture into the wall, but it will not allow the moisture to evaporate back out. Eventually water will condense on the inside of the stucco and build up inside until the wall is saturated. Many cob buildings in Europe that had stood for hundreds of years have suffered partial collapses in recent years because of new cement plasters.
Is it dirty inside?
No. Once the inside of an earthen building is plastered there is no more dust or dirt than you would find in any other building. In fact, the plasters used in cob buildings are often the same ones used on the plaster and lath walls of older wooden homes. If you want smooth white walls you can still have them with cob.
Is it cold and wet inside?
No. Cob homes are exceptionally dry and warm in winter/cool in summer. The breathable clay walls seem to pull some of the moisture out of humid air keeping the inside cooler in hot humid weather. In winter cob absorbs the energy of the sun during the day and radiates it back into the building at night, keeping the inside warm and cozy.
Is it well insulated?
Technically speaking, no. Cob is very cool in summer and warm in winter and with good design you may never need to heat or cool a cob building. If you do heat and cool your building it will hold that temperature for a long time. But, cob does not get these thermal properties from insulation. Instead cob works because of its thermal mass. Thick heavy earthen walls absorb the heat of the day and store it like a battery, keeping the inside of the building cool and comfortable. Then, when the weather is cooler they radiate that heat back out keeping the building warm.
Can I paint my cob walls?
Yes! In addition to colored plasters, you can paint an earthen wall with an almost endless variety of colors from clay and mineral pigments. There are many excellent recipes for beautiful natural paints and there are few stores and mail order businesses that sell ready made natural paints. In fact, natural paints are beginning to come back into use in conventional homes as well. Even if you are painting a conventional wall you may want to consider natural paints because latex and oil based paints off-gas very toxic chemicals, sometimes for years.
You should never paint your cob walls with latex or oil based paint for the same reason that you would not plaster with cement; it impairs breathability. You can find sources for Natural paints and recipes here (resources).
Will termites eat the straw out of my cob?
No. There is nothing in cob that the termites are interested in.
How fast can I build with cob?
How fast you finish your house depends on how skilled you are, how hard you work, how many people help, how big and elaborate your house is and, to some extent how much money you have. In warm breezy weather you can build 2 or 2 ½ feet of height on your wall each day. That may not sound like much, but if you build your walls 8ft high then you could theoretically have them done in four days. Cob is actually faster than most other building methods (including traditional stick framing and straw bale) because once the walls are built they are done. In other types of building there are multiple layers. For example, even if a framing crew framed your entire house in four days or less they would still have to add insulation, vapor barriers, exterior sheathing, drywall and paint. This process would probably take you the next thirty years to pay for. With cob, you may spend an entire summer building your walls yourself plastering them and putting a roof on them, but it could cost you as little as $5,000 so you won’t be working to pay the mortgage for the rest of your life.
How much does a cob house cost?
Ianto Evans has been quoted with the best answer to this question that I have ever heard; “How long is a piece of string?” Your cob house can cost as much or as little as you have to spend on it. The basic materials, clay, sand and straw are very cheap and you may even be able to get most of them on your own land. Huge amounts of excellent, often brand new, building materials are thrown into dumpsters every day. I have found porcelain sinks, entire boxes of marble tile, endless amounts of high quality lumber, and every other material you can imagine for free. Because cob is so malleable, it is very easy to use odd salvaged windows, doors, and wood, so if you take the time to scavenge you may not have to buy much at all. Some cob homes have been built for as little as $500 and many for as little as $5,000.
How big can I make a cob house?
Just like with any other house, you can make it as big as you can afford (in money or in time.) You probably won’t need as much square footage as you think you will though, because cob homes are usually built in rounded shapes which offer far more useable space than square buildings.
Since cob home owners generally do most of the work themselves, they usually choose to keep their houses to a modest size. One of the great things about cob is that if you decide later that you need more room you can just make an addition or build another cottage next door for the kids.
Do cob houses have modern conveniences?
Cob homes usually include all of the same plumbing and electrical wiring as a conventional home. They can also have central heat and air conditioning although the need for these is far less than in conventional house.
Is it legal to build house out of cob? Can I get a permit?
It is not illegal to build with cob anywhere in the US, but neither is it specified in the building code. In order to get a permit to build with cob, many people have first been required to employ an engineer to help with their building plans. In nearly all cases a building department will approve plans once they are stamped by a “licensed design professional.” In addition, the building code does include specifications for adobe construction and many building officials have simply looked upon cob as a form of adobe and approved construction according to that code even without the help of an engineer.
In some cases you may not even need a permit. For example, there are permit exemptions in most places for buildings under a certain size, agricultural buildings, and in some rural areas no building permits are required at all.
Should I try to build my own cob house?
For many people building their own house is a wonderful experience. The materials for building a cob house are very inexpensive and the skills for assembling them are easily acquired. For that reason cob can offer you the opportunity to build a unique home for very little money if you are willing to do the work yourself. Many of our workshops are tailored to prepare you to build your own efficient, healthy and mortgage free home. We also provide a range of services to help owner builders along the path to success in building their own homes.
For some people it may be better to hire a great deal of help and to hire a professional to oversee the construction. If you can afford to pay cash for the construction of a house and do not relish physical labor, or if you want a large and complex design, then hiring a professional may be the best path for you.
Are there professionals to help with cob building?
Yes! There are very few natural builders who act as general contractors. However there are a number of natural builders who are available to assemble and lead paid building crews, train your crew, act as consultants, project managers, or otherwise help you through the process. You can see details about our range of services here.
Can I host a workshop to help build my cob house?
Yes! Here are some of the things you probably want to know about hosting:
- We much prefer that you take a workshop before you host one. There are some exceptions, but usually taking a workshop is the best preparation for being a host.
- Workshops do not generally complete a building unless it is tiny. They do get a great jump start, involve the community, and are a lot of fun. They also afford you a great deal of professional help as part of the workshop package.
- Hosting a workshop seldom actually saves you much money over the cost of hiring help. A workshop is not a way to a house built for free. However, there are many less quantifiable benefits to hosting a workshop.