Miscellaneous Q and A's about Cob Building
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Q: I was wondering if cobbed cement would be a good building method. You could build your house slowly but would never have to worry about bugs, fire, or rain.

A: (Kelly) I'm not sure what you mean by "cobbed cement"; if you mean slowly piling up a wall by hand with ordinary concrete, I would say that it is not a good idea, because using an regular form would go much faster and would result in a stronger, more monolithic wall. If you mean stabilizing soil with a small amount of cement, and piling it up like cob, then I think this approach might work, but I would suggest doing some tests to see how the local soil works, etc.

Q: What is the cheapest thing to use as insulation on a cob house in Southern Colorado?

A: (Kelly) I also live in Southern Colorado, and it gets COLD here, so you need very effective insulation. As for what would be the cheapest, it is hard to say. One solution to insulating a cob house would be to put it in the middle of the wall, between two layers of cob. That way you can take advantage of whatever thermal mass that the cob can contribute and keep the look of a cob wall, both inside and outside. If I were you, I would use the crushed scoria that is readily available here for that purpose, since it is local, non-degradable, effective, and natural. Other possibilities include rigid foam insulation, sawdust with lime, straw...

Q: I have been "absorbing" with a sense of glee all of the info on your web site as well as what the folks at Cob Cottage Co. have to offer. I have, without knowing it, been planning for sometime an alternative dwelling. I am interested in a hybrid that will primarily be cob but I have a question about something that dates back to the 70s. Do you remember the Nearing's from Maine? I think it was Ted and Helen? An older couple that showed up in the Mother Earth Circle back then. They were promoting "Stackstone" houses or "slipform" rock walls using field stone. It basically involved a movable form that allowed for the gradual construction of a stone and mortar wall about 2 feet at a time, boxing out doors and windows as you went. I don't remember the exact terminology. Since field stone is one of the most abundant resources in our North-central PA area, I would really like to incorporate this into building plans. I am wondering if you are familiar with either of these terms?

A: (Kelly) Yes, I am familiar with some of the work that Scott and Helen Nearing promoted through their book "Living the Good Life". There is a nice article about them in the Oct/Nov 2003 issue of Mother Earth News. They were using what is called slipform masonry, just as you describe.

I know that stones are often thrown into a cob wall to fill up space, or especially at the bottom of a wall, to make it more resistant to weathering. I don't see why you couldn't slipform a combination of stones and cob. It would take longer to "set up", but it should work. To make a more comfortable house, I would suggest making the walls pretty thick, and somehow creating an insulated core, or thermal break, in the wall. This could be done with sawdust and lime (the old fashioned way) or crushed volcanic rock if available, or modern "blueboard" insulation.

Q: I am needing to build a home for my son and me. I will not have much budget to work with overall. I want to find out if there is someone who can come and build a home for us such as a cob home? How much is the cost to do this? It would be a simple design home I need a home about average 1600 sq feet? I am by Dallas Texas.

A: I wish I could tell you there was an easy way out of your predicament. Natural building techniques such as cob can be very inexpensive for owner-builders because the materials (clay, straw, etc.) are so cheap. Cob is so simple that almost anyone can learn to build it. However, it does require a lot of time and work. If you have to hire people to build your house, a cob home may not come out any cheaper than conventional. This is due not only to the number of hours of work required, but also to the fact that there are relatively few experienced professional natural builders, and so therefore you are usually paying for a lot of on-the-job training.

My suggestions would be these: First, take some natural building workshops to find out how much of the building process you can do yourself, or with family and friends. You might even find a natural building instructor willing to lead a workshop helping to build your home. If not, then find an experienced builder who you trust, and work with him or her as much as possible. Keep your home as small as you possibly can. If there are only 2 of you, you should be able to cut the size down to 800 sq. feet or less and still have plenty of room. I live in a community where the average house size is 400 sq. ft. (including for families with small kids). Try to phase your building so that a core area with the absolute necessities (kitchen, small living and sleeping spaces) can be built first, and then any additional spaces added on later when you have the resources.

Q: I'm still learning about cob, and so far I love the idea. I have one major question. What about mold? I have kid's with asthma so mold is a big deal.

A: The best strategies I know of to avoid mold problems: 1) build a home with breathable walls so you don't end up with condensation build-up; 2) use a lot of clay in your finishes (floors and plasters), which tends to absorb and distribute moisture; 3) use careful siting and passive solar design to keep your building warm and dry in the winter. Cob lends itself very well to all of these strategies. In my experience, moisture problems (of which mold is a symptom) are practically non-existent in natural buildings with breathable walls. Make sure you have good perimeter drainage and avoid moisture barriers in the floor and walls (sometimes they are helpful in the ceiling.)

You also want to do your earthen building in warm, dry weather. Where I live in Northern California, we have a 6 to 8-month window where we can do earthen building. In the winter it is too wet. If you build cob in wet weather it can take a very long time to dry, and mold can grow on the wall during that time. However, my experience is that even in that case, once dry weather eventually comes and the cob wall dries, the mold will die and not reappear.

Q: Is it sustainable? Is it good for the environment? Can it stand all types of weather conditions?

A: (Kelly) Yes, cob is sustainable, because it is a natural material without much embodied energy, and it is quite durable. Cob is non-toxic and completely recyclable. It is best kept protected from the weather with large overhanging eaves, substantial foundations, and possibly protected with a lime plaster on the exterior.

Q: I think maybe the "light straw" method might fit the needs of what I have in mind. I have this older model mobile home at my base camp. It was one of those deals where I got it for the price of having it moved from where it had been setting and onto my property. I had it set up and level, then later gutted the whole thing. The wall studs are 2"x4"s and it is pretty structurally sound. It dawned on me one day that, maybe, I (with help,of course) could start, one room at a time, packing between the stud, some variation of the cob building method. I thought about pulling out the existing wiring running throughout the studs, then later, after the walls are done, running the wiring on the outer side of the cob filled walls through proper encasement-- in a like manner to the way I've read it is often done in the construction of strawbale homes. I like the entire idea, or scenario, of using cob (light straw) for several reasons. First, mobile homes make me nervous; and I think that any method of taking a mobile home to a safer level is a smart thing to do. Logically, it seems to me that it would reduce the chances dramatically of fire spreading from one end of the mobile home to the other and engulfing the whole structure in a matter of five minutes as has sadly happened in too many cases. That's number one. Secondly, the idea seems to me that it would provide good/sound insulation value and would create a cozy atmosphere within. And thirdly, I like the idea of the electrical wiring being run on the outer side of the inside walls rather than being trapped inside the airy walls of a mobile home.

A: The idea of retrofitting your mobile home with a straw-clay mix sounds feasible to me. Straw-clay (also called "light clay" or "slipstraw") is an excellent technique for retrofitting old structures since the mixture can be packed into the cavities between the existing framing. The biggest problem you are likely to encounter is with drying. In new straw-clay construction, the forms are stripped immediately and both sides of the wall are left open to drying by evaporation for at least several weeks. In your situation, presumably you will be packing the straw-clay right against the exterior plywood or siding, leaving only the interior surface available for drying. If the drying happens too slowly, you can end up with decomposition of the straw.

To avoid that problem, I have several recommendations. First, do your straw-clay building in warm dry weather. Use a very light mixture with very little clay slip relative to straw. (Beginners are often incredulous at how light you can make the mixture and still have it retain its shape. Open all windows and doors and use fans to maximize air flow inside the mobile home. And mix borax with your clay slip as a mold retardant.

As for the electrical wiring, I don't quite understand your concern. Leaving your wiring exposed on the interior of the wall is not a good idea. It is much more likely to get nicked and create a short if it is not covered. The easiest and safest way to deal with the wiring, it seems to me, is to place it into conduit inside the wall cavity before packing with straw-clay.

C: Your answer sounded pretty good to me. I can see how decomposition would be a concern.  In my case, the cob mixture would be packed between studs and against (not plywood, but) manufacture installed metal siding, which would probably allow heat from the sun to penetrate through the metal and into the  outer-side of cob in helping it to dry.  

A: The technique you are talking about is called by several names, among them "straw light clay", "straw clay," and "slipstraw." It is not called cob. Cob is a much heavier, load-bearing material, whereas what you are contemplating, I believe, is a much lighter, straw-rich one. You will not find details on building with straw-clay in either the Cobbers Companion nor the Cob Builders Handbook. In fact, it is rather hard to find good written materials on this technique. There's a brief but informative article in "The Art of Natural Building" which I co-edited with Joe Kennedy and Catherine Wanek, and some hands-on instruction (but not much) in a brand new book called "Eco-Nest" by Paula Baker-Laporte and Robert Laporte, the best known straw-clay builder in the US.

The fact that your exterior siding is metal rather than wood is actually a liability rather an advantage, because metal is so impermeable. The straw-clay dries by evaporating out moisture. In that regard, air movement and humidity are more important than temperature. The fact that only one side of the wall will be available to evaporate out moisture will more than double the drying time. Still, I think that if you use a fairly dry mixture with borax to retard mold growth, build in very dry weather, and use fans to improve air circulation, it could work fine.

Q: I live in San Antonio, TX. I'm just learning about cob homes. Before I get my hopes up too high I need to know what the cost, for example the traditional home, would be? I'm a registered nurse and I've been approved for 125k loan.

A: It's very difficult to answer your question with any precision because so many factors effect the price of a house. Contractor-built cob homes often seem to come out in about the same price range as a more conventional custom-built home around the same size in the same geographical area. This is because although the materials cost for a cob home can be substantially less than conventional materials, the labor can be more. Some contractors are able to lower their prices in a variety of ways, including by providing some of the labor for free, either through workshops, work trade, or owner contribution. The real potential of cob and other natural materials to provide very cheap housing is when the owner takes over much or all of the job of building her or his own home. That way, most or all of the labor expenses can be eliminated. Cob is an especially suitable technique for this approach because it is so easy for inexperienced builders to learn.

Q: I have a nice house and I’m happy with it (off the grid, 100% solar &wind, rainwater tank, hot water from wood stove loop etc) but I’m very curious about building with cob. Living in northern BC, Canada, in logging country, I have more than enough trees on my 80-acre property. In order to cut down in building time and to improve the insulation value of a cob wall would it be possible to integrate a rough log structure into cob walls? I’m thinking of 3 connected hexagons built of thin, possibly unpeeled logs, simply connected by nails - without the time consuming notching. Of course there would be wide gaps between the logs, which would be filled with cob. The outside and inside would be covered with cob also, so that the logs would be invisible and completely imbedded in the cob walls. My question: will the shrinking of the logs (pine, spruce) disintegrate the cob wall? I know, cob works with cordwood, but does it work with horizontal logs in the structure? Has it been done?

A: That's a very interesting question. As far as I know what you're proposing has not been tried. I suspect it could work, but I have a few concerns. The main one is the shrinkage question you bring up. If the logs are going to shrink substantially after the cob goes on, I expect that would put a lot of stress on the cob and you could get a lot of cracking, possibly severe. Solutions would be to use cured logs or to wait a couple of years after erecting the pole structure and before applying the cob. My second concern is about not peeling
the logs. In general that's not such a good idea because it creates great habitat for bugs like termites and carpenter ants to hide under the bark and eat the wood. It also extends the drying time of the logs and makes the logs less breathable. The cob coating would probably help make a less accessible and less attractive habitat for insects, but unless the cob was applied almost immediately, that wouldn't necessarily deal with the issue, and then see above. The third concern is structural. I'm not sure what the seismic activity level in your part of BC is, but I'm concerned about the structural stability of a log structure that is just nailed together. (Unless the logs are very narrow and the nails frequent and long enough to fully pierce the poles, which sounds like a whole lot of work to me.) If the cob wall on either the inside or outside were very thick, it would provide a lot of support for the pole structure and help somewhat to hold it together in a shake. But then at that point, what is the point of the log structure at all?

Q: In strawbale building, it seems as though our cost for construction are going to be around the same as if we did standard construction since most areas require the straw to be non-load bearing. Do you have any suggestions as to how to reduce the cost of using strawbales or is COB really the only way to go cheaper?

A: There are two major ways to reduce the cost of your building. The first is to make it smaller; the second is to do a lot of the work yourself or using free or trade labor (such as through workshops, work parties, or the like.) If you are paying for all of your labor and materials, the cost of your building will be fairly similar no matter what materials you're using. I'm surprised to hear that "most areas require straw to be non load-bearing." Even here in California in a Seismic zone 4 we can permit load-bearing straw bale, so I expect you could there too with the help of a cooperative engineer. You are likely to need an engineer's assistance for a large commercial cob building as well.

Q: I want to build an earth oven for bread and pizza. The way of cobbing is my favorite. On the internet I can only find way's of making these ovens with a floor of firebricks. Is there a way of making the floor of the oven for bread baking without these firebricks and without the risk of eating the floor???

A: Rather than firebricks, I generally make the floor of my ovens using regular common red bricks. They don't heat up as hot as the firebricks and therefore are less likely to burn your bread. Another durable way to make the floor of an oven would be to use a large flat stone or piece of concrete. For more details of earthen oven construction, I strongly recommend reading Kiko Denzer's book, "Build Your Own Earth Oven."

Q: We plan to build a cob house in my friend's backyard June 23-25. I am skeptical that this can be achieved in one weekend, but my friend believes that if we have enough friends' help, we could do it. But from what I've read online, it seems as though this could take months or even years. What do you think?

A: I will be impressed if you can build a cob house in a weekend! Apart from the number of people-hours necessary to mix and build that much cob, there is the issue of slumping. It is usually not possibly to put up more than 2 vertical feet of cob in a day before the weight overwhelms the stiffness of the material. Then you have to wait for the cob to dry some before adding more. 

Q: I know you work mostly in the States, but what would be your rough estimated cost of building a 3 to 4 bedroom Cob home in the Caribbean. The majority of the materials and labor would be easily available locally, what I am concerned about is the cost of a builder to help with design and to see the project through. Can you give me a figure that I can work with while I'm in the planning phase.

A: I wish I could be more helpful in answering your query. Ianto's favorite reply to the question, "How much does a cob house cost?" is "How long is a piece of string?" I have seen cob houses built for anything from $5 to $200 per square foot. The biggest factor effecting this range is the cost of labor. I have no clue at all what the going rate would be in the Caribbean. Even within different parts of the States the going rate of labor, skilled and unskilled, can vary by a factor of 2 to 4. The other huge consideration will be the level of your own skill and expertise and how much unpaid labor you can mobilize. I would encourage you to try to find a competent builder first and have that person come up with cost estimates. That is the standard practice in custom construction.

Q: We live on the Big Island Of Hawaii and are getting ready to purchase 4 acres here. We will need to put up a quick structure and be able to live in it within 3 months. Do you think that a small cob structure would be the best choice? What do you think about using lava in the building of a cob home? How difficult is it to build your roof before the house?

A: I have heard conflicting reports on the presence of clay on the Big Island. Some people say the volcanic soils of the island aren't old enough for clay to have formed. Others say that there is nonetheless something sticky in the soil that allows for making cob. I can only suggest doing some sample batches and seeing whether you can make a fairly strong material. Lava rock ground up fine could work well as the aggregate, if any is needed. It will be very hard on your hands and feet so you will need to use suitable protection.

Cob is not usually the fastest building technique. If you have suitable materials on site it could be a good choice. I would also look into wattle and daub with bamboo or other local materials as the framework. That still assumes you can find a decent clay source. Or you could just take your lava rock, stack it up and mortar it together. If you don't have clay you could use a cement-based or lime-based mortar.

You can certainly build the roof before the walls of a cob structure, supporting the roof beams on either temporary or permanent posts. That's a good choice in hot and/or rainy areas, because the roof will protect both the workers and the cob walls from sun and rain.

Q: My fiance and I, after extensively exploring our options, decided that we want to build a cob house of our own. The fact that it is easy to learn, quick (relatively, anyway) to build, and CHEAP as well as so healthy a material is what led us to choose cob. I have at my disposal a house left to me by my Dad. It is in a horrible state of disrepair, and I would like very much to utilize it, as it holds a lot of sentimental value for me. What I would like to do, is completely retrofit this house and turn it into a green home. I'd want to use as much of what is already there for cost and environmental purposes. It has a concrete block foundation (somewhere between 2 and 3 feet high) which is in perfect shape. Also, the roof was redone with sheet metal about ten years ago, and it still in fabulous shape. I'd like to basically gut the house and use cob, or light straw clay to fill it back in. I'm wondering which of these two methods would work best for the situation (or maybe if I should try straw bale) if I went at it like this...

Supposing I took down all of the walls and ripped out the floor, but left the structural components and the frame intact, I was thinking that I could fill the cinder block foundation, in layers, with stone, gravel, and sand. I would install an earthen floor on top of this. This (theoretically) in conjunction with the existing concrete block foundation, might be strong enough to support the additional weight of the much thicker cob wall without having to add additional foundation structures, right? Would this support a straw-clay wall that is thinner and lighter, if not cob? Or maybe build a dry stone foundation running along the inside of the concrete block foundation wide enough to support whatever wall system I care to use, and then fill in the remaining space? Does it even make sense to fill in the foundation like that? I really want an earthen floor, and that would add thermal mass to my structure. Also, would insulating within the foundation be necessary? I'd think not with the huge thermal mass created by doing it this way. Also, I wouldn't think I would need a moisture barrier, since the stone/gravel/sand layer would raise the floor so high from the ground (2-3 feet).

As for the walls... In theory I'd build up around the existing framework, so that the roof is supported while I build. I thought straw-clay might be the best for this, both for the exterior and interior. Would moving the walls inward be necessary, or a good idea, since the eaves of this house aren't designed with earth walls in mind (rain protection), or would lime plaster solve that problem? Could I use cob in this same way, building within the existing framework? It wouldn't have to be load bearing then.

The roof I'd be using as is. I would redo the ceiling and insulate with green materials, but essentially the roof would go unchanged, supported by the original wood frame, but with cob/straw clay (or a combination of the two) built up to it.

A: First I want to say congratulations on your building plans. I wonder how much practical building experience you have been able to amass? I would think it would be very helpful for you to work with some other people, either in a workshop setting or as a volunteer on a natural building project, to get a more realistic assessment of how much time and work a project like this entails, and to hone your building skills if you have not yet done so.

Without knowing quite a bit more about your situation and your site, it's difficult to answer most of your questions. If you are in a place with very cold and/or cloudy winters I would recommend using light straw-clay over cob for most of the exterior walls to increase insulation. In that case you may also want to build out your wall framing thicker - for example, to create a 10" wall cavity instead of 4" or 6" that you are most likely starting with.

The roof overhang may need to be extended (that would probably be quite a bit easier than moving the foundation and walls inward), but I can't say without knowing more about how much rain you get and how much wind,and from which direction the wind comes, how much tree cover or other wind protection you have, and how long the existing overhangs are. One possible solution would be to add porches and/or attached greenhouses to the sides of the house from which your wind-driven rain comes.

I would not recommend building a structural cob wall thicker than the foundation. If the cob is not intended to be structural (you're leaving the structural framing in place) it could be OK. The biggest question then would be about stability during earthquakes. Placing a very heavy cob wall on a cinder-block foundation not designed to support so much weight could be a problem. Are you in a seismically active area of the country? How high will the walls be, and how thick, with how much taper?

I think your floor strategy would work well. It should give you very good drainage and I agree that you would not need a vapor barrier. However it sounds like it would require an enormous amount of material. If your stone and gravel aren't available for free that would also correspond to substantial expense. It seems to me that you could fill the foundation only part way, and cut the foundation down where the doors will be.

One final comment. I understand that this house has sentimental value for you, and I also applaud your desire to reuse existing structures and materials. There are so many unhealthy, poorly-designed structures out there that we need to start converting to work better for people. That being said, you can very often get a better finished product, for little or no additional time and labor, by tearing down what you've got and starting from scratch, re-using as many of the materials as possible. This sounds to me like it might be such a case, given the problems with both the foundation and roof. If in addition to those issues the building is not oriented towards solar south or not sited in a good solar location, I would strongly recommend starting over. It would be sad to go to all the work of building your ecological home and end up with a house that was not well insulated, not structurally sound, had moisture or drainage problems, or could not be passively heated and cooled.

Q: My question is regarding cob, adobe house building...if you merely "pile up" the mixture to form a wall, is there a "best" height you should pile up at a time? And how long would it take (to dry) before you could add to the height of the wall (I have land in the desert of Arizona where it is dry and can get hot.)

A: The drying rate for a cob wall is highly variable, depending not only on local climate and weather but also on your mix, wall thickness, and other factors. In the Arizona summer, the cob is likely to dry out just about as fast as you can get it onto the wall, unless you have a large crew working on a small building. Your challenge is more likely to be keeping the top of the wall moist so you get good bonding between layers without cold joints. For a much more complete discussion of drying issues (including ways to speed up or slow down the drying time when necessary) see "The Hand-Sculpted House."