Appropriate Uses for Cob

Michael G. Smith has a background in environmental engineering, ecology, and sustainable resource management. In 1993, along with Ianto Evans and Linda Smiley, he started the Cob Cottage Company, a research and teaching group focused on reviving and improving traditional forms of earthen construction. He is the author of The Cobber's Companion: How to Build Your Own Earthen Home (Cob Cottage Co., 1998) and co-author of The Art of Natural Building: Design, Construction, Resources (New Society, 2002) and The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage (Chelsea Green, 2002). He teaches practical workshops and provides consultation to owner-builders on a wide variety of natural building techniques, site selection, and design. He lives in an intentional community in Northern California. His informative website is www.strawclaywood.com.

Questions and Answers

Q: What are the weaknesses of cob?

A: Do you mean the structural weaknesses, or the general weaknesses of the system? If you mean structurally, then cob (like all masonry materials) is much stronger in compression than in tension. That means that although they can withstand a great deal of pressure from gravity, cob walls can fail when subjected to large shear forces, such as earthquakes.

If you mean generalized weaknesses, here are a few. Cob, like other earthen building systems (as well as straw and wood) can be damaged by excessive moisture over time. It is important to keep the walls of the building from becoming saturated with water. In cool or moist conditions, cob dries slowly, which can make construction of a cob building very slow under those conditions.

In my opinion, the specifics of the building project (including for example climate, seismic conditions, building size and usage, the local availability of different materials, ease of access to site, social and political factors, to name a few) and the desires and skills of the builders and/or clients determine which building system or systems are most appropriate. There are many specific circumstances under which I would not recommend using cob because another material or system would be more appropriate.

Q: I'm new to this cobbing and I am not sure about how to see through an idea I have for my school garden. My garden consists of 8 raised-beds and we would now like to include a seating area in the center of the garden for reading and observations. Is there a way to make it using straw bales for the form and then covering it with clay? What do you mix with the clay? Will it withstand the cold winters of Wisconsin? Is it expensive? Can I use clay from a farmers field? Do you know of any awesome resources?

A: There are many ways to make a bench using natural materials such as clay and straw. One way would be to make a mixture of clay, sand, and straw, called "cob", and sculpt it into a bench of whatever size and shape you like. Another would be to use straw bales and plaster over them with wither, a clay-based or a lime-based mixture. If the bench is not covered with a roof, a cob bench will probably last quite a bit longer than a straw bale one, as the bales will rot out if they get too wet. For either cob or bales, moisture is a much bigger concern than cold. If you want the bench to last more than a few years, you should put a roof over it and/or cover it with a weather-resisting finish such as a lime-sand plaster. A good book to read for further details about mix ratios, building techniques, etc. would be The Hand-Sculpted House by Evans, Smith, and Smiley (available above).

Q: I and my family will be moving to Mozambique in southern Africa in December. I am really interested in building a home with cob. However, I have some questions over tropical climates and cob. The climate, broadly speaking, is a four month wet season which is the summer and the rest of the year is the dry season. I have heard that a high mass structure such as cob will not do well in a tropical climate. What do you think?

A: The issue is not so much whether the cob will survive the climate (cob does well in very wet regions, if adequately protected), but whether it will make the most comfortable and efficient house in a given climate. In hot, moist tropical zones, temperatures tend to stay high at night. Massive materials such as cob lose their cooling properties under those conditions since the "thermal battery" doesn't get discharged at night and the mass stays warm all the time. In moist tropical climates like those, the usual strategy is to make a lightweight, low mass building high off the ground and with as much ventilation as possible. I don't know much about the climate of Mozambique, but it sounds like a desert with monsoons, similar to parts of the American Southwest. If that's correct, then cob should make a good choice. In those climates the air humidity is basically low all the time except when it's actually raining, so the air cools off at night and heat stored in thermal mass can be discharged. There's one easy way to find out whether cob would be suitable where you are moving. Look at the vernacular architecture in the region. Cultures that have existed in a certain locale for a long time generally find the most efficient solutions given the materials they have available. If massive earth walls are typical, then they probably work well. If not, then there is probably a good reason.

Q: How is the dew point condensate managed within a cob wall? In cold climates this would surely be an issue... but long lived cob in such places is a testament to its irrelevance when done correctly. Can you explain this? How is it possible to avoid the vapor barrier...?

A: No doubt a dew point does occur within cob walls, especially in cold climates, as you suggest. Yet as you also note, cob walls have held up fine for many centuries, even in Northern Europe. I believe this attests to the large capacity of earthen materials to absorb, distribute, and release moisture over time without ill effects. Experience suggests that cob can absorb a great deal of moisture, both from weather and atmosphere and from isolated accidents such as plumbing failures. As long as the cob is allowed to dry out again, there is rarely a problem. When the saturation point is reached, there can be a sudden and spectacular collapse, but that seems to happen only when either the cob is actually sitting in water for some period of time or coated with a non-breathable covering such as cement stucco. So preventative measures are fairly straight-forward: good drainage to prevent flooding, and a breathable surface coating. As well as, of course, an adequate roof. In places where you have a lot of wind-driven rain, you may want to protect the exterior with a weather-resistant but breathable plaster, such as lime. But that is really more to avoid cosmetic damage and slow erosion than because of moisture concerns.

Your question points to one of the big differences between cob and straw bale. Straw, as a biological material, is very susceptible to decomposition by microorganisms, given the right conditions, which include moisture. Once the moisture level gets to a certain point, the straw will begin to rot. Cob is made mainly of geological materials - sand and clay - which are not subject to digestion by microorganisms. The clay tends to preserve the straw in cob because it is more hydrophilic, and thus draws the moisture away. I have seen samples of cob 200 years old, in rainy New Zealand, where the straw inside was still yellow.

Q: Is it possible to combine Rastra block walls (for a basic rectangular house) and do cob detailing on the outside? I want to build an off-grid passive solar home that will last 500 years... but I want it to look great, too, with the neat details you can do with cob. Is this do-able, and if so, any suggestions (like surrounding the panels with chicken wire like some do with bales).

A: An interesting idea. It is possible to get cob or other earthen materials to adhere to almost anything. If you only want to build up a thin layer of earthen plaster, all you need is a surface that is sufficiently rough. I'm not sufficiently familiar with Rastra to know whether it meets the requirements. Certainly you can apply an earthen plaster over a standard brick wall. If the surface is too smooth, you will need to create additional "tooth," either by scratching or etching the surface or by painting on an 'adhesion coat,' made, for example of sand and glue watered down to a paint-like consistency.

If you want to add a thick layer of cob to your wall to allow for the sculpting of niches or other high-relief work, I would recommend making your foundation that much wider and building up the cob from the bottom. It would also help to leave some sort of pins (framing nails, for example) sticking out from the mortar joints to improve the connection between the Rastra wall and the cob. Applying the cob on the interior rather than the exterior surface makes a lot more sense from both a thermal and a weather-protection point of view. Let us all know what works!

Q: I am wondering if it is possible to cob over interior concrete block walls. This is an existing house with unfinished concrete block walls we would like to use natural plastering and cob to create the natural earthen made look. Is there anything special that we need to apply to the concrete first.

A: Yes, I'm sure that what you're proposing is possible. The trick will be finding a good way to attach the cob to the concrete. If you are doing only a fairly thin earthen plaster, you can first apply an "adhesion coat" made of flour paste or white glue and coarse sand. When it dries, that will give the wall enough "tooth" or texture to hold onto the plaster.

If you want to apply cob more than an inch or so thick (which might be nice to mask the harsh square lines of the concrete block and introduce some sculptural elements) you will need to create some sort of an armature to support it. I would use some sort of concrete bolt or screw (many types are available at your local building supply store). Drive them in until firmly anchored but leave them sticking out a few inches. Then you can wrap galvanized fencing wire around the heads to make a sort of skeleton for your cob elements. The length and spacing of the bolts depends on how much weight of cob you will be attaching, and on whether the cob is supported on the floor below or whether it is free-floating attached only to the concrete. But in general I would recommend something like a six inch spacing both vertically and horizontally.

Q: I am planning to build a cob home on the beach and am curious how practical it would be to plaster my shower wall with an earthen plaster and imbed seashells over the entire wall. If I do this, what would be the best way to seal it?

A: I don't know for sure how that would work out, but it's worth a try. Sounds beautiful! I would try a small sample first and leave it out in the rain for a while or wet it with a hose a bunch of times to see how it holds up. You could try sealing the earthen plaster with linseed oil. That should work fine on the earthen plaster, although it may get somewhat gummy on the shells and discolor them. Also we are finding that linseed oil tends to develop mildew over long periods of time. There are a number of other "non-toxic" oil-based sealants available commercially, but I don't have much experience with any of them. A more successful approach might be to imbed the shells in a lime plaster. I would suggest making several samples, using both lime and clay plasters and sealing them with various sealants, then do some rigorous water testing before you commit to doing your whole shower that way. Please let us know how it goes!

Q: What about using cob for the walls of a double-chambered compost toilet and also for graywater beds indoors and outdoors and sinks? What is the best way to waterproof these?

A: Earthen materials like cob do not tend to be durable when subjected to lots of moisture. I wouldn't recommend using cob in direct contact with the ground, except in the very driest of climates. I have heard of people making cob sinks and bathtubs and sealing them with linseed oil. I imagine that if you re-oiled them frequently they would last for a while, but I don't know how long.

Q: Where would I purchase earthen plaster? If we took all the dry wall and insulation out of the wall could we put straw where the insulation went, then chicken wire, then mix straw into the earthen plaster and place on top of that?

A: There is a company now manufacturing earthen plaster and selling it dry mixed in bags - just add water and apply. The company is called American Clay (www.americanclay.com) and I have no personal experience with their products. Most people mix up their own earthen plaster from clay that is either found "in the wild" or purchased dry in bags, with sand and fine fiber (such as chopped straw) added. A great resource for earthen plasters is "The Natural Plaster Book" by Cedar Rose Guelberth and Dan Chiras. I would not recommend applying it over chicken wire - metal and earth are very dissimilar materials that don't bond easily. Also the chicken wire will probably not give you the stiffness you need for easy application of plaster. A better choice for the situation you're describing would be reed mat, which is available at hardware stores for landscaping purposes. Another option would be bamboo, small saplings or thin milled wooden lath, running horizontally and stapled or nailed to your studs.

Also remember that compared to synthetic insulation materials, straw does not give you great insulation per inch of thickness. The most you can expect is about R-2 per inch. Depending on where you live, simply filling your wall cavity with straw may not give you adequate insulation for a cozy and efficient home. You might either want to build the cavity out to greater thickness or find another material with more insulation per inch of thickness, like wool for example. Another simple option is cotton batting. A company called Bonded Logic makes cotton batts called Ultratouch, which are similar in insulation value and application technique to fiberglass batts, but much more healthy and pleasant to work with.

Q: I live in the Jordan valley, 200 meters below sea level. What is better to use for building: adobe or cob?

A: Cob and adobe have very similar structural properties, as the components used to make them are practically identical. The main difference structurally is that well-built cob, because it lacks all the "cold joints" or seams intrinsic to adobe, should fare substantially better during an earthquake. Other factors contributing to your choice of one technique over the other include weather, availability of skilled labor, and your personal aesthetic. It is possible to build with cob in cool, damp weather unsuitable to adobe, although I don't imagine that being an issue where you are. It may be that there are local builders trained in the use of adobe but not in cob. If so, it might be simpler to go along with the local vernacular rather than having to train a work force in a new technique. Of course, if you plan to do most of the building yourself, that also becomes less of a concern. Cob, because it uses no forms, is very easy to sculpt into rounded and organic shapes. If you like that look, cob may be for you. Or you could use a combination of both techniques, taking advantage of the speed of erecting adobe walls (assuming you can buy pre-made adobe blocks) and the sculptural properties of cob within the same building.

Q: I am currently developing plans for a cob house outside of Waco, Texas. Since the winters are mild and relatively dry I am considering building with a full berm on the north side and with only partial berms on the east and west sides (straw bale for the north and west walls, earthen floors, interior partitions, and thick plaster). Does this plan make sense, or is there a better way to build a cob structure in this climate?

A: Not to say that it couldn't be done, but I don't know of anyone who has successfully built a bermed cob structure. Trapped moisture inside the wall is death to cob, so you have to be extremely careful about drainage and waterproofing. The same goes for straw bale. I would generally not recommend using either material below grade. The exception could be in a desert climate with extremely well-drained soils, where there is no chance at all of moisture infiltration from wet soil into the below-grade part of the building. A safer option would be, if you want to berm, to use a material (such as concrete or stone) that will not be damaged if it gets wet up to and a little bit past grade, and then switch to cob or straw only above that level.

If the winters are so mild, I question the value of berming the building at all. I imagine that your intention is to keep it cool in the summer. However, a very high-mass wall material such as cob, in combination with earthen or other masonry floors, will achieve the same effect with less potential complication.

Q: I have obtained funds for an urban sustainable garden from my local council. However our ambitious plans for curved raised beds have been turned down on grounds of cost. Is it possible to make the walls for the raised beds out of Cob? Would it need to be treated in any way to prevent moisture damage? Would cob be strong enough to resist vandalism?

A: (Kelly) Cob would not be a very good choice for raised garden beds because it is basically mud reinforced with straw and allowed to dry; when it gets damp it will return to mud. Even with a plaster of lime cob is best protected by a roof to keep it dry.

Perhaps a better choice that should be quite inexpensive would be the use of earthbags for this project. Either the individual polypropylene bags, or continuous polypropylene tubing can be filled with earth and formed into lovely curving shapes for garden beds. Two strands of barbed wire are usually placed between the courses to help keep them in place. The bags need to be plastered to protect them from the UV in the sun, but they are unaffected by moisture, and even if the soil inside gets damp, they will not generally deform. Either a lime plaster or cement stucco is rather durable, but of course nothing can resist all sorts of vandalism.

Q: I plan to build a 5/8 geodesic dome house using straw bales and cob on the exterior. My biggest worry, as you can imagine, is waterproofing, especially as I live in a fairly rainy climate. Do you think that earth plaster and linseed oil would do the job? Do you have a formula for such a plaster? How many coats should I use and how often should I renew the protective layer?

A: I'd like to do whatever I can to dissuade you from your plans to build a cob and straw bale dome. I think that is a very dangerous undertaking, especially in a rainy climate. All building materials expand and contract with changes in temperature and humidity. I don't believe there's a plaster in existence that would keep your dome leak-proof for more than a few years. Once the plaster cracks and water gets through, the cob and especially the bales are capable of absorbing enormous amounts of water. Both will become very heavy and at the same time lose their rigidity and compressive strength. This is a disaster waiting to happen. Ken Kern, the pioneer of owner-building in the 60's and 70's, died in an experimental earthen dome that he built and plastered with concrete stucco. He was sleeping in it the night of a huge rainstorm, when enough water finally got through the cracks in the plaster and into the cob walls to snap the concrete structural reinforcements. Please learn from Ken's death and don't put yourself in a similarly dangerous situation.

You may be saying, yeah, I know, but I will do the necessary maintenance and replace the plaster every few years, or every year if I have to. That may be so (although it sounds like a set-up for a huge amount of ongoing work for the rest of your life), but can you be sure that whoever else might be living in the house after you're gone would understand the danger and keep up on the maintenance? Are you comfortable gambling with the lives of all the building's future inhabitants?

So I would say that plaster as a waterproofing surface is out. The next possibility would be to use a flexible waterproof membrane such as EPDM to protect the outside of your dome from moisture. Unfortunately, you would thereby encase your earth and straw building in a material with no vapor permeability, opening yourself up to a whole other set of moisture problems resulting from condensation between the earth/straw and the membrane. Toss that idea out the window.

The final option would be to build your dome, then build a waterproof roof structure over it. This is the most feasible idea, but still not really a good one. First of all, it's a big duplication of effort, since one of the main points of the dome in the first place is to avoid having to build a separate roof structure. And secondly, if your roof were ever to leak for any reason (which eventually happens with most roofs) you would be right back in the first scenario, with potentially waterlogged earth or straw over your head, in the most dangerous possible location.

Incidentally, I'm not alone in my distrust of domes as a building system. LLoyd Kahn, who published the 70's classics "Shelter," "Domebook," and "Domebook 2" took the latter 2 books out of print after coming to the conclusion that geodesic domes are impractical. If you look around the world at vernacular building traditions, you will only find earthen domes in extremely dry climates. There's good reason for that.

Q: I'm thinking of building a cob sculpture of a woman. I live in Ireland and am wondering about the weather here. If she was about 10ft high how long would she take to dry, and would a lime plaster be sufficient to save her from the rain? I was thinking of a hazel structure underneath. Is there anything I need to be aware of / watch out for? Do you know of anyone who makes sculptures from cob? How long would it last in rainy old Ireland, and could you point me in the direction of some good books?

A: Your cob sculpture project sounds exciting! It's very difficult to predict how long cob will take to dry. If you have high temperatures, low humidity, and wind, your sculpture could be dry in a week or two. If the weather is cool and/or damp, drying will be much slower. I've seen wet cob sit for months during a rainy Oregon winter without getting dry.

The answer to your lime plaster question is also not straightforward. My guess is that, no, lime plaster will not be enough protection. It might hold up fine for a few years, or it might not. Lime plaster is porous and will absorb water if there is no roof to keep rain off the sculpture. If that water then freezes in the plaster or the cob beneath, it will crack off the plaster. Here in Northern California, a much milder climate than yours, I have not yet found a finishing treatment that will keep cob in good condition through more than 3 or 4 winters. The only sure-fire way to make your sculpture last longer than that is to protect it with a roof.

There are no books that I know of specifically on cob sculpture. I would recommend reading one of the general cob construction guides, either "The Hand-Sculpted House" by Evans, Smith, and Smiley or "Building with Cob" by Weismann and Bryce. Either of these books will help you make a good cob mix and walk you through the basic construction process. I think the idea of building an armature of hazel sticks is a good one. I would recommend first trying a much smaller and less ambitious project - perhaps a quarter-sized scale model, to try out your mix and techniques.

Q: I am living at a sustainable demonstration house at the University of Montana in Missoula. We are rehabilitating a large garage building on this site to become a meeting space and community center. I traveled through Denmark this summer visiting ecovillages and saw quite a few beautiful cob buildings, so I am particularly interested in putting in a cob floor and potentially some cob interior walls in this garage. The first issue for us is the existing concrete floor. I read that you thought a cob overlay of a concrete wall would work without too much problem, do you think the same would be true of building our floor on top of concrete? Any moisture issues we might want to consider?

A: When installing an earthen floor over concrete, moisture is going to be your major concern. If the existing slab is not well drained, moisture can work its way through the concrete and get into the earth, causing it to get soft and/or grow mold. Sealing the slab first with some sort of concrete sealer might be advisable, and improving the perimeter drainage if it seems inadequate.

Comment: Here's a picture of a Truro oven that we recently completed here in Truro, Nova Scotia. Just thought you might like to see it. It turned out beautifully!

Q: I would like to construct a cob hearth in my house for my wood stove to sit on. The vertical portion of the hearth will also be acting as a bathroom wall - which I thought would be a nice way to get the heat from the wood stove to transfer to the inside of the bathroom. I live in NH. It is already October and I am already using the wood stove. I could turn the stove off for a week to do the construction but then I am concerned that running the stove so close to new cob will cause the cob to dry too quickly and crack. Should I just scrap the project until spring? The stove is now sitting on durarock...ugly but functional. :)

A: It is possible that the cob could crack if you get the wood stove going really hot before it is dry. It is also possible that the cob could take a really long time to dry, especially on the bathroom side, and that you could get mold growing on it. Cob drying time is affected not only by temperature but also by air movement and humidity. When people are doing indoor cob projects I generally recommend leaving all of the windows open and putting fans in the windows to get the moisture out of the house. You probably won't want to do that in winter in New Hampshire. So for all of those reasons it would be prudent to wait until the weather is warm.

Q: I like cob for mortaring cordwood. What material/proportion do you advise?

A: Yes, in my experience a cob-like mixture works well as mortar for cordwood masonry. I would recommend a mixture of clay soil, sand, and either chopped straw or horse manure. The fine fiber will make the mortar easier to point (make smooth) as you go, without long messy straws hanging out that you would need to trim later. The exact proportions depend on your clay soil. You could start with a mix that is 2 parts clay soil, 2 parts sand, and 1 part fiber. If it cracks as it dries, add more sand. If it seems soft and crumbly, add more clay.

The only place where a cob-like mortar will not work well is where it gets wet frequently. So at the bottom of the wall, if you don't have a high foundation to elevate the cordwood masonry wall above the level of rain splashing off your roof, you might consider using a mixture with cement or lime as the binder.

Q: To me the most intriguing dwelling shape is the tipi. Would a tipi shaped cob structure be possible or maybe a truncated cone with a flat roof.

A: It's theoretically possible to build a tipi-shaped cob structure. However it would be somewhat of an engineering challenge. Cob walls are very heavy. If they are vertical, they support themselves, as they are also very strong. But at an angle like a tipi wall, you would need a very strong tension structure to keep all that weight from falling over, and possibly also an even more massive foundation as a counterweight. My recommendation would be to consider a vault or dome shape, which are compressive structures well suited to earthen construction. Or if you want your tipi shape then to go with a lighter wall system like wattle-and-daub. From a practical perspective, I would find a tipi-shaped home difficult to inhabit efficiently. There would be a lot of wasted space around the perimeter that isn't high enough either to stand upright in or to receive furniture. And if you live anywhere where earthquakes are possible, it would be very dangerous to have so much earthen mass overhead. This is a problem with vaults and domes as well as a tipi-shape, and explains why these forms are typically found only in seismically stable areas.

(Kelly) Michael Smith suggested that I send you a link to some pictures of the earthbag house that I built in Colorado because it has somewhat of a tipi feeling inside. There is a description of this house at earthbagbuilding.com

Something else I might add to Michael's comments about building a cone (or vault or dome for that matter) with cob is the danger of failure if the cob gets saturated with moisture from rain. Even with an impervious layer of plaster or paint this is a danger.

Q: I have been looking into building a cob house and want to know if it is possible to make a cob house with an earthen roof and then push dirt around the structure to make it sort of a hill? I am concerned about moisture build up. I want to be able to grow wild grasses native to Oklahoma. I hate to sound silly but the closest thing I can find to what I am speaking of is a Hobbit House. I am concerned with the weight pushing on the walls.

A: Covering a cob house in earth doesn't sound like a good idea to me for 2 reasons. The first is structural; earth piled up against a wall can exert an enormous amount of sideways pressure on the wall. A cob wall might or might not be able to withstand that pressure; it would depend on the qualities of the site soil, the strength of the cob, and the height, thickness and shape of the wall. But calculating ahead of time whether or not the wall would be strong enough would be hard to do since so little engineering testing has been done on cob.

Secondly, it's important for the longevity of a cob building that the walls be able to "breathe." That means that water vapor needs to be able to find its way out of the wall to prevent moisture building up inside the wall. If too much moisture builds up in the cob, the wall can lose its strength and collapse. If you were to pile earth up against the outside of a cob wall, you would have to seal the outside of the cob wall to prevent moisture wicking from the damp earth into the wall. But whatever you used to seal the wall would also prevent it from breathing. So I don't think this is a good strategy except maybe in an extremely dry place where it almost never rains.

If you're attached to building a "hobbit house," you could do so using concrete as your wall material. There are a couple of books out there that will tell you how, including "Earth-Sheltered Houses" by Rob Roy. But if you want to build your walls out of cob, stick to a living roof but let the walls breathe.

Q: I have recently been given an old mobile home, which is water tight but cold and tinny. I would love to cover it in cob, rather than rip it all down to start again. Would it be possible to cover it in cob? I've seen people apply cob to walls covered in chicken wire....would it be a similar solution?

A: Cob usually refers to a structural earthen wall made out of clay soil, sand and straw. You can take the same (or similar) ingredients and apply them in a non-structural way to an existing wall surface. I would refer to that as an earthen plaster. You can build the earthen plaster out as thick as you wish to provide thermal mass and/or sculptural texture to a flat, industrial surface. You would probably want to attach some sort of lath to the existing wall before applying the earthen plaster (unless the plaster coat is very thin, in which case you could simply paint on a mixture of sand and glue to provide "tooth.") There are several things you can use for lath; my favorite is reed mat which you can buy in rolls at the hardware store. Run the reeds horizontally and secure the mat to the wall with lots of screws through the wire lashings. Chicken wire could also work as lath if you stretch it tight, but is less pleasant to work with.

I'm presuming you want to plaster the inside of your mobile home with earth. That could increase temperature stability, reduce mold problems due to condensation, and improve the look and feel of the interior. Just to be sure you understand what you will NOT achieve this way: you will not get a breathable wall and you will not significantly improve the insulation. A mobile home will always be somewhat cold and tinny no matter what you do to it; it's in the nature of the materials the structure is made of.

I do not recommend applying earthen plaster to the exterior of the mobile home. Although it is technically possible, it would be a large project and will leave you with ongoing maintenance, in exchange for very little improvement to the functionality of the building.

Q: My partner and I are thinking about building a solar and wind powered cob sauna and steam room for my parents. What would you recommend for the interior of a steam room that could be in constant use? Would sealed tiling affect the breathability of the structure and encourage damp? I understand it will probably have to be a hypothetical answer but how long do you think the traditional lime plaster would last in a typical 40 degrees Celsius humidity?

A: It so happens that I've built a cob sauna and plastered the interior with lime/sand. After 13 years of weekly use there is no sign of plaster degradation or moisture problems, so that's what I'd recommend for you. Sealed tiles would also probably work, as long as the exterior surface of the cob walls are left breathable. Important details to consider are going to be the floor shape and material (you don't want puddled water to soak into the cob walls) and the ceiling, catching and diverting all the condensation before it gets into the walls from above.

Q: I am in the planning stage of building my container home in South Florida. The idea of living in a metal box kinda wigs me out so I wanted to use cob for my interior walls to soften it up. Do you think that would create any mold problems if I were to use denim insulation?

A: I'm not sure what exactly you have in mind; whether you are talking about freestanding cob for interior partitions or an earthen plaster surface over your insulated exterior walls. Either one sounds feasible, but keep in mind that no matter what you do on the inside, you will still be living in a metal box. Metal does not breathe and so I would expect lots of condensation, which can lead to mold. It might be possible to mitigate this with a high-tech air-exchange system, but I don't know of any way to address the potential moisture and mold issues using passive techniques and natural materials.

Q: I am thinking of building a cob home underground, best described as a hobbit house. How would one go about keeping the cob from damage after it's buried?

A: I would not recommend building with cob below grade. To last a long time without moisture problems, it's essential that cob walls be able to release any water they have absorbed, ideally to both sides (inside and out). That is obviously not possible if you waterproof the exterior of the walls to prevent them from becoming soaked by absorbing moisture from the soil. If you are in an extremely dry location (i.e. the desert), you might be able to get away with the cob releasing moisture only to the interior of the building. Otherwise I would suggest you find another building system, such as gravel-filled bags.

Q: I am building a 10'X12' cob building above ground. I would also like to build a larger COB BUILDING UNDERGROUND, 20'X20'. How would I do the roof and how deep do I need to go to maintain upper earth stability?

A: Building with cob underground is not recommended. To avoid long-term moisture problems which could possibly lead to collapse, the cob walls need to be able to release moisture to both the interior and the exterior of the building. This is nearly impossibly to achieve underground, since whatever material you use to protect the cob from absorbing moisture from the soil will also prevent it from releasing moisture. I would instead recommend using earthbags, earth-filled tires, or a non-earth-based wall system for underground construction.

Q: Is there a way to build a cob shower and finish it to withstand the water?

A: There are a few options. I have made a cob shower and mosaic tiled it using conventional tile adhesive and grout with waterproofing additives. After over ten years of continual use there are no apparent problems. You could also probably use cement stucco with a sealer. Or tadelakt, which is a Moroccan lime plaster technique polished and sealed with olive oil soap. In any of these cases, I would use a very breathable finish such as clay plaster on the back side of the cob wall, to prevent moisture buildup inside the cob.

Q: I have been dreaming of building a cob home for my son and me for years and finally have the time and means to do so. My concern is this: what happens to the cob walls in the bathroom when taking hot showers with a lot of steam and also the steam that comes through the vent to the outside from running the dryer

A: Cob and other clay-based materials have an enormous capacity to absorb water vapor, store it until the relative humidity of the surrounding air drops, and then release it again without any damage. This is very different from absorbing large amounts of liquid water as can happen if there is a flood, inadequate drainage, or a roof or plumbing leak. Basically, moisture in the air is not a problem, whether it is natural humidity or use-generated. This vapor-absorbing capacity is one of the huge advantages of natural earthen materials over many conventional building materials. It only works if you surface the walls with a vapor-permeable finish such as clay or lime-based plaster. On the other hand, if you plan to use cob for the walls of your shower stall, you will need to select a very waterproof finish, such as tile, to protect it from the liquid water.

Q: I live in the Pacific Northwest in Washington. It's not very cold here but often wet. I want to build a cob oven this winter which seems fine since I will be lighting a fire in it. Will it cure properly or should I just wait until summer?

A: It's always safest to wait to do your earthen building during dry weather. In cool, damp weather clay will often just stay wet indefinitely and grow mold, and the straw in the mix can begin to rot. That being said, an oven is perhaps the one project it might not be crazy to attempt in the wintertime. As you say, you can light a small fire inside to facilitate drying. The question would be how long you would need to wait before moving the support from inside the oven. The oven can't dry effectively as long as the sand remains inside, but on the other hand it needs to dry enough to bear its own weight before removing the sand mold. In short, this is a risky proposal but if you wouldn't be crushed if the oven were to collapse, then by all means go for it. You can always build it again in the summer if it doesn't work the first time.

Q: I am currently researching a project, and was looking to find out your thoughts. I want to be as eco-friendly as possible, recycling and saving the environment as much as possible. I have studied Earthships, eco-domes, and cob to an extent. I have been looking to build a dome hotel. Because I am in Wisconsin, straw is abundant... I am considering whether I can use a combination of tires (as mentioned for Earthships) with a cob roofing. I was wondering your thoughts on this

A: I would recommend staying away from domes. Except in desert conditions, it's extremely difficult to keep domes from leaking, no matter what material they're made of. All building materials expand and contract with changes in temperature and humidity. The expansion and contraction leads to cracking and water infiltration. With a dome, essentially the entire structure needs to be treated as a roof which is typically the most expensive and technically challenging part of a building.

In an area like Wisconsin, with very cold winters, good insulation is going to be essential to keeping your building comfortable without a huge expenditure of energy, which results in both high cost and high carbon emission. The best natural materials for insulation are the lightest materials: straw, for example. Neither earth-filled-tires nor cob have very good insulation. Earthships function without insulation by being recessed deep into the earth where the solar heat that penetrates can be stored in the thermal mass of the earth. The same strategy won't work in a cloudy climate like Wisconsin; you will need much more insulation there and less thermal mass. 

Cob is made of clay, sand and straw. If it gets wet and stays wet, the clay gets soft, loses its strength, and eventually washes away. The straw rots out and loses its ability to bind the rest of the material. Cob is not at all suitable as a roofing material. Earthen buildings in rainy climates always have overhanging roofs made of another materials that doesn't get soft when it gets wet.

Q: I would like to build a greenhouse using cob... for a variety of reasons but largely due to cob's thermal mass. I was wondering whether this is possible structurally and if there would be any differences to building to take into consideration?

A: There are no special structural issues I’m aware of regarding cob greenhouses. I have built 2. The main issue we had with both of them is that they didn’t let enough light in for optimal plant growth. Ideally plants should have light coming from all directions, not just above and South. If you kept the cob walls fairly low with glass above, that would probably work fine.

The other issue to consider is water. Greenhouses tend to be very wet places, and you want to think about where that water will run so it doesn’t damage the cob. Perimeter drainage is a good idea, and an impermeable foundation to get the cob up away from the wet ground is critical, just as it would be in a cob home. If you will be spraying a lot of water close to the cob walls I would use a lime plaster for the interior finish.

Would a wallipini style- slightly underground greenhouse work with cob or not as you mentioned the importance of water impermeability? Also, I had the idea of building an earthen oven within the structure of the greenhouse as an idea to shelter the oven from rain damage as well as it providing extra heat. However I was wondering whether I would have to provide flue holes in the roof for the smoke to escape. Alternatively I have heard of the idea of using flue residual heat to heat another structure e.g. a heated cob bench but I was unsure how this works. Lastly, my location is in England and its nearly winter and I was wondering whether its feasible to build during this season (in terms of the cob being able to dry out) 

You certainly wouldn’t want to have the cob walls below grade. You could use another material, such as stone or concrete or even gravel-filled bags below grade and then transition to cob above grade, but that seems like a lot of work. Probably simpler to just use the same material all the way up to where the glass starts, since that should be fairly low anyway.

You definitely need to vent the flue of the oven (or stove, more likely) out of the greenhouse some way or other, or you would choke yourself out with smoke and blacken the interior of the glass or plastic. You could either  run the flue through the roof or out through the sidewall of the greenhouse. The latter approach would work best with a rocket mass heater, which is what you’re referring to with the heated cob bench. You can find details on how to build one of these in the book “Rocket Mass Heaters” by Ianto Evans and Leslie Jackson. Building cob is best done in warm, dry weather.

Q: Could cob be added to the interior of a uninsulated metal building to add thermal mass/insulation or would condensation be a issue? I'm looking for a cheap earthen way to insulate a barndominium.

A: I wonder exactly what you have in mind. You could either build a fairly thick, self-supporting cob wall inside of your metal building, or you could apply an earthen plaster directly to the metal walls. Both would be feasible. The former would add a huge amount of weight to the structure and you would need to engineer for that and make sure the weight is adequately supported. One advantage is that you could leave an air gap between the cob and metal walls to prevent any possible condensation issues. It’s difficult to get an earthen plaster to stick to metal, but it can be done. I don’t know how much of a problem you would have with condensation over time - probably depends a lot on your climate and how well the space is ventilated. One thing I would mention is that either way, cob is a very poor insulator - something like R-0.25 per inch of thickness. So if it’s insulation you’re looking for, this is not a good material to choose.

Q: I want to restore an old cement sheet house in the mountains west of Sydney. The cement sheet contains asbestos, so it's recommended the outer cladding be left in place to avoid contaminating the site. The climate has extremes from 40C+ in summer to -10C in winter & can be very windy & wet or very dry. I'd like to use slip-straw or light clay straw to insulate internally between the existing timber framework, with clay plaster & clay paint on top of this. Externally I'm thinking of using cob with a lime plaster. My concern both internally & externally is that the slip-straw & cob would have to sit against the impermeable cement board. Should I leave a gap between the slip-straw or cob mix & the cement board for damp to run down? I could put a channel at the bottom for condensation to run off. Or is it OK to put the slip-straw & cob directly against the cement board?

A: This is a rather difficult problem. Light straw-clay is typically used only when and where it can dry to both sides. Otherwise drying is very slow and you can have problems with mold growth and decomposition of the straw. If the weather is very warm and dry (i.e. no rain for months at a time) and your wall is not too thick (say 15 cm or less), you might be able to get away with it only being able to dry from one side. I like your idea of leaving a gap between the straw-clay and the cement board, but I’m not sure how this could be achieved in practice without resulting in a very weak wall system.

Another possible solution, that I have become quite excited about lately, is rice hulls and lime. Rice hulls, hydrated lime, and water are mixed together and installed as an insulating system similar to straw-clay. Advantages: better insulation than straw-clay, extremely mold-resistant, can be installed in wet weather, very easy to mix and install. Disadvantages: higher embodied energy for the lime production; requires permanent lath to hold the hulls in place since they are much more crumbly than straw-clay; only appropriate where rice is grown. (1 part dry hydrated lime; equal part by volume of water; equal part by weight of rice hulls)

As for the exterior of your wall, I don’t understand the value of using cob outside the cement cladding. I suppose you could reduce your use of lime by using a base layer of earthen plaster. But you will be introducing more dissimilar materials, more possibilities of failure (especially as water is likely to get through the lime layer and into the earthen layer below) and more steps than necessary. I think you would be better off going straight to lime plaster over the cladding, probably with the use of some sort of mesh to improve connection and reduce cracking.

Q: I am assisting with a cob oven building workshop (have built two basic ovens for myself). This person has gone ahead and built a metal platform for the base: I am concerned about starting the sand/clay base on top of this: I historically have used sand/clay/laid the empty bottles/filled with clay slip/then sand and fire bricks etc. Will this process 'slip' on top of the metal table on his metal frame? I may be over cautious; the finished oven will have a protective roof built.

A: My primary concern about building an earthen oven on a metal base (assuming it is strong enough to support the weight) is that metal expands and contracts a great deal as it heats up and cools down. This movement could transfer to the earthen structure and cause the oven to crack and fall apart much faster than it otherwise would. I think if you are able to build up a sufficient thickness of insulation and thermal mass on top of the metal base, there shouldn’t be much of a problem. I would recommend that you install the layers you would normally use on top of the platform: start with a layer of cob or sand/clay, then bottles, then sand, then bricks.

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