Appropriate Uses for Cob
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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: If you build a cob structure with thicker walls than normal, can you plant short rooted plants in the wall to add color on the outside? Like some succulents that multiply but have a short root so as not to harm the rest of the structure. Just a thought. It might be interesting to see a growing home.

A: In order to keep succulents growing on your cob walls, I imagine you would have to water them, at least occasionally. In the long term I don't think that would be good for the wall. Penetrating roots could contribute to softening the surface which would accelerate erosion. As you point out, you could build the wall extra thick to accommodate this. But a wet wall is also a colder wall, stealing heat from the building via evaporative cooling. If the building needs to be heated, a better option might be to let ivy or other climbing vines grow up it.

Q: The house I am visiting lacks any soil, having an entirely concrete courtyard. Can we construct 2 foot walls to enclose beds for planting gardens or will the watering of the beds within deteriorate the enclosing walls? Is there any way to work with cob in this case? Lining the base and sides with gravel, and creating small spaces for draining along the base?

A: If I understand right, you mean to build short, narrow cob walls to enclose raised beds for gardening over a concrete slab. There will be 2 threats to the durability of these cob enclosures. The first, as you point out, is erosion of the cob due to watering and rain. The cob can be made to last much longer if it is sealed with oil of some sort - as many coats as possible. We use linseed oil here in the U.S. I don't know if that is easily available where you are. The second threat would be from saturation by water held inside the cob enclosure. As you point out, this can be addressed by building in many small drain holes and filling the bottom of the bed with gravel. Even if you do both of these things, the cob beds will have a limited life span. My guess would be a couple of years to a couple of decades, depending on the quality of the cob, the oil, and the weather. But I think it's worth a try. Let us know how it goes.

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: I am wondering if the panhandle area of Florida is adequate for cob construction? I currently live in Ohio but was planning to move to the panhandle area of Florida in a couple of months. I looked at some soil maps and found that there may be clay soils where Florida borders Alabama and Georgia. Obviously it is sandy along the panhandle's coastline. My concern is that Florida may be too humid and have too much precip. to accommodate cob construction.

A: I don't know much about the geology or climate of that part of the world. And I don't know of any cob houses in that vicinity, although I do know an experienced cobber who is currently living in Tallahassee. If there is clay, there is no reason why you can't build with cob there. Cob has been built successfully in very wet places like British Columbia and Wales. If you get very frequent rain you may want to put up a big tarp over the site during construction, or build the roof first. As long as it is carefully designed and built (good high foundation, adequate roof overhangs, protective plaster if necessary) it should last a very long time. However, that doesn't necessarily mean that cob is the most suitable building material for that area. High mass materials like cob can be great at passive cooling, but only if there is some way to cool them down. In a dry climate like the high desert, even when daytime temperatures are very hot it still gets cold at night and the heat absorbed by the cob during the day can be discharged. In a hot moist climate where nights are warm, high mass materials lose their passive cooling abilities. You may need to augment with mechanical cooling. And a lower mass building system (like straw bale or wattle-and-daub for example) might be more efficient. But I don't know the climate of the panhandle well enough to say for sure.

Q: We would like to build a cob house in France but we are not sure if the soil is suitable for this way of building. Would you have some information on which parts off France you could use this method of building, or where else in Europe?

A: (Kelly) Cob can be done almost anywhere in the world, and was very common in Europe, especially England, historically. All it takes is a combination of sand and clay soil, along with some chopped straw to make good cob, and these ingredients can be found in many locations. About 20%-30% clay is ideal, and this can be mixed from different sources. It should not be very hard to find the right soils to build in most areas of France.

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: Can a cob home be built with a basement of concrete block? Before I buy a book I want to make sure it is even possible to do this as I want a fall-out shelter and food storage area under the house. I also wonder if this building material will be practical given our mild winter climate.

A: Sure, a cob house can be built on a foundation of concrete block. Just like with any other building type, the foundation should be engineered to withstand the loads from the building and from any earth movement. With its high thermal mass, cob is ideally suited to places with mild winter climates. It also withstands very humid conditions better than some other natural building types such as straw bale. The most up-to-date book on cob design and construction is "The Hand-Sculpted House" by Evans, Smith, and Smiley. It will give you a lot more answers to your questions. (Although admittedly basements for cob are not covered in much depth. For various reasons having to do with passive solar design, drainage, and seismic stability, cob builders usually choose to put their buildings on top of the ground.)

Q: I am pretty darn poor (about 12,000 a year) and I have two daughters. I have been able to acquire a 1/4 acre of land and as you can image I need to build cheaply. I am earth conscious and wanted to build with cob. My kids would probably love to mix the cob "for awhile". Could you suggest a starter room style for me to start out with and be able to add to later as I am able? I was thinking round and we can all kinda sleep in the same room like a dorm and I was thinking kinda cubby type areas stacked like bunks for the beds. The biggest problem I am coming up with is combining the kit/bath area it seems to take a lot of space! Also we live in Oklahoma so we have high winds and the temp gets over 100 and down to zero. Any suggestions?

A: Under your circumstances, starting with a very small building and adding on as you have time and energy makes a lot of sense. You will want to plan for that addition from the beginning, making sure that things like the exterior site grade, foundation, roofline, interior floor levels, windows and doors will all accommodate the addition gracefully. Generally a round or curved shape is the most difficult to add on to. You could still have part of the building curved if you want, but include a straight wall to make the addition easier.

Another suggestion I have for your starter cabin is to consider making it two stories. Building up often saves money as the foundation and roof tend to be the most expensive (and technically difficult) parts of the buildings. You could put living/cooking/washing areas below and sleeping above. This makes heating the space easier, as you can heat the lower level and control the passage of heat up into the 2nd story (or loft) as you need it. A higher initial roofline also simplifies later additions, as the add-on roofs can be tucked under the original one. The only real disadvantage of a 2-story is that it requires more wood and more carpentry than a single-story cob building.

A final suggestion would be that you consider making some or all of the exterior walls out of straw bales rather than cob. Bales go up quicker than cob and provide much better insulation, which could be important in your extreme climate.

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 cob, perhaps with stones pushed into it, could be used to wall up a cave dwelling dug on a hillside?

A: Under most conditions, it's best to avoid building an earthen wall like cob in direct contact with the ground. If the ground is ever wet, the cob will absorb that moisture and get soft. If it gets completely saturated, it can collapse. So only in the very driest conditions (desert) would I recommend what you are suggesting. You're much better off using stone with mortar for that purpose.

Q: I'm living in a collective in northern Oregon and we have an unused Airstream trailer in the back yard. We plan to turn it into a meditation space. My idea is to tear out the cheap, tacky interior and build walls out of cob on the inside. I don't believe this trailer will be mobile any time soon, and would become more of a semi-permanent fixture, as there are blackberry bushes growing around the outside. I'd like to know if this would be stable, or how high I could build up the walls on the inside. If you've ever seen an Airstream you know that the ceilings are rounded. In your experience, is it safe to build a rounded cob ceiling? I have lots of 2x4 wood. Would it be necessary to place studs on the walls in order for the cob to bind to the walls, or would it bond well enough anyway?

A: An intriguing challenge- how to remodel a travel trailer into a healthy, natural space? From my own experiences with trailers (including several remodel attempts) I know that there are some pretty serious issues to overcome. One is that the "cheap, tacky interior" is actually structural! Those trailers are generally supported by a structural sandwich made of the exterior aluminum siding, very lightweight wooden framing (usually 1x2s) and the interior particle board. Cutting out the interior surface significantly reduces the strength of the structure. I presume that part of your intention is to increase the interior thermal mass. It's a conundrum how to do that without reducing the useable interior space too much. Cob walls, to be self-supporting, need to be at least 4" to 6" thick. That of course also adds a lot of weight to the trailer and would virtually guarantee that it would never move again. Then there is the issue of the ceiling, which as you note is a challenge in itself. My proposal would be to apply earthen plasters to the existing interior surfaces, rather than building structural cob walls. That would be much simpler and safer. There are simple techniques for getting plasters to stick to unnatural materials, generally using either an "adhesion coat" made of flour paste and sand or a natural fiber lath made out of burlap. I would use 2 coats of plaster, a base coat between 1/2 and 1 inches thick and a finish coat about 1/4 inch thick. Then if you really need additional thermal mass I would add it to the floor, either by installing a layer of bricks or pouring an adobe floor. Another huge challenge in those trailers besides the lack of thermal mass is the lack of insulation. One way I have solved that problem in the past is by putting straw bales (or other insulation) on top of the trailer roof, then building a new lightweight roof over the top of that. At some point with this sort of a remodel you have to ask yourself, was it worth it? No matter what you do you are going to end up with a small cramped, low-ceilinged space with poor passive solar features that is not
breathable and therefore is prone to mold (especially in a climate like yours, unless you're on the east side of the mountains). Although I applaud the attempt to reuse a funky structure which already exists, it might be almost as simple, and may result in a much more pleasant result, to start from scratch.

Q: We have just purchased a stone and cob cottage in France. We propose to remove part of one of the cob internal walls and my question is can the cob we remove be reconstituted and used on the rest of the house (i.e. outside walls under the roof) to make repairs fill holes etc. If so how best can this be reconstituted. Eventually we intend to have the house re-cobbed outside but need to make some weatherproof repairs in the short term and seems a shame to waste anything original!

A: You should certainly be able to re-use the cob that you are tearing out for repairs. Find a way to grind it into powder (ideas include pounding it with a sledgehammer and driving a heavy vehicle over it on a hard surface) then add water and mix as you would regular cob. Depending on the quality of the original mix, you may want to add extra clay or sand, and you will certainly want to add extra straw. The British have developed excellent techniques for repairing old cob cottages there, which vary depending on the nature of the damage. The Devon Earth Builders Association has published several excellent pamphlets on the topic.

Q: My husband and I want to build our first home. We have been looking at prefab homes where there is a wood frame, exterior walls, siding and roof (no insulation or drywall). We would like to use a recycled cellulose insulation covered by cob for our interior walls. We are thinking of using the cob as a type of earthen plaster to finish the inside face of our exterior walls; we will already have framed walls made of wood with an exterior siding. As for the cellulose, they now have blocks of recycled paper insulation much like the standard pink insulation. We are also going to use cob for some other things such as permanent furniture and islands in the kitchen. Will this work? Do we need anything between the cob and insulation. We are living in Northern Ontario (lows of -45).

A: To apply earthen plaster (or any other kind of plaster) you need a fairly firm, resistant surface. You won't be able to plaster directly onto the cellulose bats, but would have to install some sort of fairly rigid lath over that. This could be sticks (either milled or unmilled), bamboo, reed matting, or expanded metal lath. Any of these could be nailed or screwed to the inside of your framing. The first coat of plaster is likely to crack due to movement in the lath, so plan on applying at least 2 coats. I wonder whether you have considered straw light-clay as an alternative insulation material. It will not give you as good insulation per inch of thickness as your cellulose, but you could fairly easily frame your walls out to make them thicker by the addition of 2x4 studs on the interior. The straw-clay is firm enough that it will take earthen plaster directly without need of lath.

Q: Is it possible to build a cob home that resembles a typical home of America (i.e. drivit or stucco)?

A: You certainly could make a cob home that looked like a "regular" stucco house. I don't know exactly what the point would be, other than to blend in to a conservative neighborhood. Most people choose cob because of its unparalleled sculptural potential. Building a cob house is like sculpting your dreams out of clay. Why would you go to all that trouble and end up with a flat, straight, wall? Maybe you could camouflage it on the outside to look like regular suburbia and indulge your imagination on the interior.

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 work at a boarding school and would like to build a sweat lodge. I would like the sweat lodge to be more permanent than just blue tarps. I was wondering if cob and then a plaster outer covering would be a workable idea. Do you have any suggestions as to what might work better? Also would a sapling frame covered with chicken wire be enough to support a dome structured cob sweat lodge.

A: It is certainly possible to build a sweat lodge using cob or another earthen building technique. Cob itself is not usually built with a reinforcing framework. It is possible to make an unreinforced dome out of cob using the corbelling technique described in "The Hand-Sculpted House" or "The Cobber's Companion." This will take a while, as you have to let it dry as you go to prevent collapse. Another approach would be to weave your sapling structure, as you would for a traditional sweat lodge, and then use a wattle-like infill system. If you can find enough flexible sticks, you can weave the whole structure like a basket and then smear over it a sticky mixture of clay, fiber (shopped straw or manure) and sand if necessary to reduce cracking. This is called "wattle and daub." Where long, straight, flexible sticks are scarce, I prefer a technique called "clay wattle." You take handfuls of long straw, coat it in a thick clay slip of pudding consistency, and then weave those long straw-clay sausages into your sapling frame.

However you build your structure, you will then have the problem of waterproofing it. There is no simple solution that I know of. In rainy climates, earthen structures have always depended on a rain-shedding roof to protect them from weathering. Over the years, we've been experimenting with various kinds of weather-resistant plasters. There is none that I can say for sure would work in your situation. Cement stucco tends to crack and let water through that way. Lime plasters are less brittle and easier to repair, but will let some moisture through by capillary action. Also, if there is much expansion and contraction in the structure (which there may be because of high temperatures), any kind of plaster will crack. On outdoor cob structures like benches I've had best success with a clay finish plaster treated with several layers of boiled linseed oil, as
you would seal an earthen floor. However, the oil drastically reduces the permeability of the plaster (that's really the point.) In a sweat lodge, you release so much water vapor inside the structure, that I would have concerns about condensation inside the earthen wall, trapped there by the oil, potentially weakening the earth to the point of collapse. Maybe that wouldn't be a problem. But commercial linseed oil also gives odd harmful fumes (especially when hot), which may make it incompatible with the healing function of the lodge.

If it were me, I would probably stick with the traditional approach to a sweat lodge, covering it with blankets, hides, etc. which can be taken inside when it rains. Part of the beauty of the lodge is that it's so quick and easy to rebuild as the sticks decay. Or I would build an earthen sauna with a roof to protect the walls. That's what I have at my home. It lacks some of the magic and intensity of the lodge, but it requires a lot less maintenance and is much easier to heat. Or I would build a wattle lodge and not attempt to seal it from the weather, planning to rebuild it as needed. Its life could be extended quite a bit by covering it with a tarp when not in use, but there you are right back where you started, depending on a short-lived plastic tarp.

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: Has anyone ever used COB for a commercial building like a B&B?

A: I hear rumors about large cob buildings being planned in the USA, but I don't know of any firsthand. There are certainly commercial-scale cob projects underway in the UK, including an entire school being built by Cob in Cornwall (cobincornwall.com)

Q: I'm planning to make a cob house, but because I would be doing a lot of the work by myself, I want a bit more of a structure underneath it. Do you know of any combo geodesic/cob houses, and is this a crazy idea? I love geodesic domes, but they use so much wood -- this seems like the best of both worlds.

A: I'm not sure what problem you are trying to solve by combining cob with a structural frame. One of the big advantages of cob is that it can be built without any framework, so it can be done by people without carpentry skills or access to wood. Once you have a framework, it seems to me that there would be faster and easier ways to enclose it. I would also be very concerned about all that weight overhead if you were to enclose a geodesic dome with cob. And then there are the waterproofing issues, which are always tricky with a dome, but the consequences of leakage could be severe with an earthen wall. So for all of those reasons I really don't recommend the cob/dome combination.

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: What would it take for you to come and turn our walls into cob walls?

A: I wish I had a magic wand I could send you through the mail. Unfortunately, the process is somewhat more complicated than that. I would recommend reading "The Hand-Sculpted House" or another cob building guide which will explain the steps to building a cob home. It is unusual to retrofit a conventional building into a cob building - generally the foundation and roof are not built in such a way as to make that transition feasible. What you most likely could do, however, would be to apply an earthen plaster on the inside of your conventional structure. This would give you some of the feeling and other benefits of an earthen wall without a major structural remodel.

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'm getting ready to build a house into the side of a hill and was thinking of using cob. Any suggestions would be great. Also I'm wondering what would be good for a floor; I'm trying not to use concrete. If any of your books or videos would help let me know witch ones I should purchase.

A: Unless you live in a desert, building earthen walls below grade is probably not a good idea. You could use a more waterproof material, such as stone, to build up to grade, and then switch to cob at that level. Even so, be very thoughtful about how to deal with the drainage issues to prevent moisture from the ground from making its way into your home.

An excellent natural substitute for concrete floors is poured adobe, which can be troweled smooth and finished with linseed oil to make it hard and water resistant. You can find complete instructions in my book "The Hand-Sculpted House." The same book is also a step-by-step guide to building a cob house.

Q: We live in SW NE and own an old home built out of hallow red clay bricks with one inch or less of concrete covering them. We have recently gutted the whole house and would like to remodel the interior with cob walls. We plan on sealing the exterior wall on the outside to prevent moisture from entering the building. Would it be a problem to build cob walls next to our exterior walls on the inside of our house? We are concerned that the cob may not be able to breath on one side, would this be a problem and if so is there a way to overcome the problem and still use cob? We have also thought about encasing our existing exterior walls in cob. Would having both the inside and outside covered in cob, with our existing bricks and concrete in between, present a problem structurally?

A: I would not recommend building cob up against a wall which is not vapor-permeable. In fact, any exterior wall which is not vapor-permeable can lead to serious problems with interior moisture build-up and sick house syndrome. So please be very careful about how you seal your exterior walls.

I'm not quite sure where you are planning to put the cob walls, but if they came in perpendicular to the exterior walls and touch only at one end, I don't think you will have a problem. Other than that, I imagine you are talking more about an earthen plaster over your existing wall material, rather than a thick, load-bearing cob wall. If so, except for the moisture problems mentioned above, I would not be concerned. A fairly thin layer of earthen material, or several thin layers build up slowly over time, should be able to dry just fine with air exposure on only one side.

I don't know what would be the advantage of an earthen plaster on the outside of your bricks and concrete, other than aesthetic. Also if you have any driven rain you are setting yourselves up for a lot of long-term maintenance. I would think that a lime-sand plaster would be a much better choice.

Please be sure to assess the structural capacity of your foundation, floors, etc. before adding lots of extra weight to your 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.