Plastering or Waterproofing Cob
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Q: I am currently researching plaster and rendering materials for cob walls specific to heavy use, public spaces. The surfaces have to be strong enough for kids to climb on and durable enough to endure the spilling of hot beverages, food, etc...I have been hearing about Sodium Silicate, commonly known as waterglass, to seal for water, etc. Have you had any experience with waterglass? I also have questions about Casein for durability, and a good source of recipes.

A: Finding durable finishes for cob has been an ongoing quest. Kiko Denzer has had good results with waterglass to protect cob and earthen plasters from the weather. I have never tried it myself, and don't know what it does in terms of enhancing hardness. See Kiko's article in issue 14 of the CobWeb (available from Cob Cottage Co., 541-942-2005). I have had pretty good results with lime-sand plasters, although they can take quite a while to get fully hard. Another thing I've seen work great is a clay plaster with several coats of linseed oil applied afterwards. It is extremely hard and water-resistant, like an adobe floor. This is probably my best recommendation for your situation. Casein does increase the durability of soft earthen plasters and clay paints. I don't know how it would hold up to a lot of wear and tear and moisture. Good luck!

Q: I want to let you know that there is an environmentally safe soil stabilizer on the market named Earthbind 100 that may be a good alternative to asphalt emulsions used to stabilize and waterproof cob. As you know, asphalt contains many of the carcinogenic polyaromatic hydrocarbons. Earthbind does not. It is a mixture of paraffin resins and tree lignin. Earthbind has been thoroughly tested and does not contain any organic compounds that are considered toxic. In addition, Earthbind was been extensively tested regarding the toxicity on fish, mammals, and plants and it considered very non-toxic. I believe that it will stabilize cob better than an asphalt can because of the resins. I would like to send you free sample. For complete production information please visit our web site at www.enviroad.com.

A: Thank you for your information and the kind offer. I would certainly like to know more about your product and to try it out. I would also like to let you know that I have never used asphalt emulsion to stabilize cob, nor has anyone I know. It is not recommended practice for the reasons you mention and many others. Cob is always built without any stabilizers mixed in. A weather-resistant plaster made of lime, or earth stabilized with linseed oil, is then applied where necessary. Perhaps your product would work in place of linseed oil as a water-resistant exterior finish? Could you please send me a sample and a complete list of ingredients?

Q: I am planning on building a chapel with strawbale turned on it's side with 4 inches of cob on either side. Is there any reason this wouldn't work? Also, what kind of insulation would you recommend for the roof?

A: What you are proposing sounds a lot like a technique that is being promoted by the Cob Cottage Company under the name of "balecob". The current edition of their newsletter, "The CobWeb" contains 2 articles on it. You can get a copy by calling 541-942-3021. I know of no reason why your idea wouldn't work, as long as drying conditions are very good (dry, hot, windy weather) so that the bales don't stay wet for long. I wonder, however, why not just stack up your bales and put a very thick earthen plaster on both sides. That sounds a lot simpler and quicker to me.

As for insulation in the roof, there are many options. One of my favorites is the recycled cotton batting called Ultra-Touch, which has been available in California only for the last couple of years. For other options, check out chapter on roofs in "The Hand-Sculpted House" or the section on natural insulation options in "The Art of Natural Building."

Q: I am about to start working on an old farm house in France that has some of the walls made with Cob. There are a couple of cracks that need to be repaired. I am demolishing an old tractor shed also made with Cob. Can I utilize some of this to repair the walls on the house. How would I go about doing this? When the repairs are done, how should the wall be finished. both internally and externally? Internally, I was thinking of battening, insulating between the battens and then plaster boarding to allow painting or tiling. Is this OK? I have heard that laying new concrete floors with a damproof membrane can damage cob walls. is this correct and if so what is the alternative?

A: Here in the US we have only been building with cob for about 15 years, so we have yet to develop much expertise in cob repairs. However, that expertise does exist in Britain, and probably in France as well. There's a new book out of England called "Building with Cob" by Adam Weisman and Katy Bryce, which has a chapter on restoration that will answer all of your questions about repair techniques. Cob walls are generally rendered or plastered with an earthen or lime mixture. It is not recommended to use a non-vapor-permeable surface such as tile or commercial paints. These sometimes lead to major structural problems as a result of moisture being trapped inside the wall. If you use battens with insulation and then plaster board, make sure the space between the cob and the surface is very well vented to prevent moisture accumulation. The same sort of problem can result from the use of damproof membranes or vapor barriers in the floor,
although it is generally less devastating that in the walls. Make sure that the drainage underneath the floor is excellent, by using drain tile and several inches of drain rock underneath the slap. Earthen floors are generally considered more compatible with cob walls than concrete.

Q: The house in question has stone along the base of the walls to about 2 feet high. As this separates the cob from the soil, will it be Ok to install a damp proof membrane under a concrete floor? Would you recommend injecting a damp proof course between the mortar in the stone work?

A: This would probably not cause any problems for the cob walls. However, you could still end up with a cool, damp building unless you provide additional drainage. Depending on the kind of stone used (whether or not it is porous and can thereby wick water up from the ground) you might seal the top of the stone stemwall before putting cob on top.

Q: I am building a cob bench in wet Vancouver, BC and am looking for an alternative to adding a rain shelter. Have you tried a hydrated lime plaster with linseed oil finish? Would that still breathe? The bench is on a rubble trench (with drain tile) and two layers of drain rock in bags for a stemwall. Decent drainage I think, non? Are you aware of any other waterproof and breathable coatings that would work in this rainforest environment?

A: This is the question of the year. The best luck I have had with unroofed cob benches is to use an earthen plaster and then several coats linseed oil. Oil makes a non-breathable finish but this is less problematic for an outdoor bench than for the wall of a house. The oil will need to be renewed at least yearly - more often if it is in a very sunny location where the UV will break down the oil. In a shady location, you may get mildew growing on the oil during wet whether, which turns the surface black and also breaks down the oil. But with frequent re-oiling it should last a while. How long, I don't know. Depends on a lot of factors such as the quality of your clay, carefulness of detailing with plaster, etc. But probably 10 years - maybe more.

Lime plasters are water resistant in that they don't get soft when wet, but they are not impermeable. I did a really beautiful and time-consuming fresco over lime plaster on an outdoors cob bench. The next winter, water soaked into the plaster on the seat of the bench and froze, severely cracking the plaster. You may be onto something with oil over a lime plaster. I don't know much about the compatibility of those materials. We have one wall here at Emerald Earth where someone put linseed oil over a lime plaster. That reduced the breathability to the extent that we ended up with mold problems both on the inside and outside surfaces of the wall. Again, that will be less of a concern on a bench, where permeability to water vapor is not so critical. But you may well end up with black mildew stains on your plaster.

When it comes down to it, I don't know of any way to plaster a cob bench that is guaranteed to protect it in a rainy climate. You're much safer with a roof. But if you want to experiment with other kinds of finishes, by all means go ahead, and let me know in a few years how it looks.

Q: I have an outbuilding that is presently unfinished and in need of siding over the exposed plywood walls. Can I use cob as an exterior siding? I live in Portland, OR, a cool, rainy climate.

A: What you're referring to is an earthen plaster, rather than cob. Cob is a structural mixture of clay, sand, and straw, used to build walls. The same ingredients can be used to make an earthen plaster. Earthen plasters can be made to stick to virtually any surface (although for non-earthen materials like plywood or concrete considerable surface preparation is sometimes required). However, earthen plasters are not particularly weather resistant. If your walls get much rain running down them, I wouldn't recommend earth as an exterior finish.

Q: I noticed that cob homes in England that are 200 and 500 years old do have stucco outside to protect the cob. Maybe that is a more recent solution to other problems though?

A: I think there's a misperception about the stucco on old cob buildings in Europe. The traditional protective plaster on the exterior and usually the interior as well was a mixture of lime putty and sand - no Portland cement. Lime plasters are waterproof in the sense that they do not get soft when soaked, but they are also very breathable. They are traditionally maintained by periodic applications of limewash. Some cob buildings have recently been re-plastered with cement stucco, but this very often causes serious moisture issues in the earthen wall, and has in several cases contributed to collapse of ancient buildings.