I would add that in extreme climates with lots of exposure they built with rock - so if you do have any local hard material (low rot woods or stone) this may be your answer for that exposed wall.
WATER RESISTANCE for Cob - In order to make earthen plasters and paints more water resistant, several strategies may be used. Binders like linseed, walnut, palm, or tongue oil, or sodium silicate (aka potassium silicate, aka waterglass) may be painted on final finish surfaces, and/or added in small amounts to the entire mix of one or more layers. Also, burnishing with steel trowels or smooth stones can mechanically align and compress the surface to close up pores and slow down absorption. Both of these strategies can be combined to create a burnished and stabilized surface that will likely out perform most anything else. Tadelakt is an ancient lime plastering method that combines the chemical stabilization of lime with thorough burnishing, as well as an oil/soap treatment to the final surface that seals all pores. Such a surface, if properly maintained, can remain totally waterproof indefinitely. Other similar methods exist that use only clay binders and linseed oil in many layers to build up a waterproof ‘linoleum.’ However, these methods are arguably out performed by the application of a shingled covering like wood, ceramic, or rock. The addition of these extra surface materials takes more time but is longer-lasting with less maintenance needed. Earthen materials can also be compressed to such an extent that they become nearly waterproof. Compressed earth blocks, for example, can resist rain on the vertical surface for a great number of years before eroding to the point that they even show wear. Depending on the amount of water exposure, it may take many decades before water would cause them to fail structurally. http://www.onecommunityglobal.org/plaster/
planer shavings, saw dust, sticks or wood shavings/chips. Horse or cow poo. Any kind of natural fiber like cotton, jute, flax, ramie, sisal, hemp, coir (coconut) or dissolved paper. Mineral fibers would do it too.
longer fibers are as a general rule, desirable because they provide tensile strength to the mix. Sand (or other aggregates) provide load strength.. Take a handful of sand and crush it, it resists, but pull that handful apart and it shows no resistance whatsoever.. Do the same with a handful of straw (fiber), very little resistance to crushing but difficult to pull apart. Generally speaking, the longer the fiber, the better it's woven into the wall (whatever thing you're making), the more tensile strength they provide. As I said before though, it becomes less critical if you're building small things, like benches and so on.
As to proportions, that all depends too. What do you want it to do? Where is (this particular) mix located? Questions like that.
When I'm building arches and larger overhangs, I add LOTS of straw to the mix, though when I'm building near a stove I'll use very little straw but add more sand. Is the thing your building going to experience a lot of downward loading, or will it be pulled in multiple directions?
There IS NO one-true-mix.. You've got to fool with the stuff and see how YOUR clay does with YOUR sand(aggregate) and YOUR straw (fiber). You may find that your local soils need no added sand for a good every-day cob. You may find that the kind and amount of roots in your soils make up for the lack of local straw. No way to know without trying. So make tests with different mixes, let them dry out and test each one to destruction. Don't be afraid to try crazy shit, you may be surprised at the results.
Failure #1: Mixing earthen and lime plasters on a wall surface. This is perhaps the most common mistake that I see over and over again. People choosing to use earthen plaster for the scratch and brown coats and a final, “durability coat” of lime. The problem here is that what you have is stronger plaster over weaker plaster when in reality, you want it the other way around: weaker plaster over stronger plaster.
If you consider all plaster work over the last say…thousand years, one thing holds true no matter what material you use. The second coat has more sand in it than the first coat and the third has more than the second. That makes the coats “weaker” as they move away from the wall. This is important because plaster moves, as do homes. If the weaker plaster beneath a strong lime finish coat can move more than the finish coat, you will ultimately get delamination between the two coats which will lead to eventual plaster failure. By laying weaker plaster over a stronger finish coat, it will always be able to move at least as much as the coat beneath it. This keeps the plasters well bonded and eliminates the high risks seen in the opposite application. Cracked cement plaster.
Failure #2: Trapping moisture in the wall. There are two main ways to create this problem. The first is to use a plaster than does not breathe well. For some reason, the use of cement in plaster is still celebrated by some builders. I do not understand this at all. We know that cement based plasters don’t breathe well and we know they are more prone to cracking than lime or earthen plasters. Sure they are stronger, but who cares when they will eventually cause your walls to rot. DO NOT USE CEMENT BASED PLASTERS on a bale home. That is as easy a fix as any.
The second way that moisture gets trapped in a wall is something I see all the time. People decide to use earthen plaster on the home’s interior and lime (or even worse: cement) on the exterior. Those materials all have a different rate of permeation. Let’s consider the most common application scenario: earthen on the interior and lime on the exterior. In any given hour, the earthen plaster will allow 10 units of moisture to enter the wall. During that same hour, the lime will allow 7 or 8 units to exit the wall. The remaining 2-3 units are stuck in the wall and will continue to build up in the straw, leading to moisture trapping issues which are the cause of bale decay.
I very often hear people complain about lime plaster being a bad choice because “they have heard” lime will cause rot in the bales. NO, THAT IS NOT TRUE. What is causing the rot is the overloading of moisture in the wall due to the uneven plaster moisture rates. Yes, there will be rot behind a plaster coat made of lime in this situation, but the lime is not to blame, it is the combination of materials that were used.
To prevent this problem you can either use the same material on both sides of the wall, or you can build up the interior coat to slow down the rate of movement through the wall. This is my favorite option. In the same example, we could simply build the interior earthen coat to 2” and leave the exterior lime coat at 1 1/4”. The added thickness on the interior will slow down the moisture movement through the wall such that the lime can release as much as the earthen will allow to enter in the same time period. Simple fix.