Questions regarding earthen plaster render in mostly dry climate versus lime plaster render.

We will be constructing a small 12' diameter round earthbag building in the spring.  I am doing as much research as I can, but find two differing opinions at the below websites concerning exterior plaster.  

Our original plan for exterior plaster/render was for clay/sand/straw plaster to fill in crevices between the earthbags, and then after curing, add a couple more layers of same plaster, and finally a couple coats of lime plaster to finish off.  I read recently that unless you have actual cob walls, or clay plaster thicker than 4", the lime plaster will not stay on because the earthen plaster shrinks and expands and although the lime plaster breathes it does not expand like the earthen plaster, and within a year or two, the lime plaster will crack and crumble away.  This same website mentions that you could mix the earthen and lime plasters but should be tested as the mineral content of clay may cause a chemical reaction with lime.  Then the other website state that you can lime plaster on top of earthen plaster to weatherproof or make water resistent as the lime allows moisture in and out as it breathes.  We do not get much rainfall here, but some snow.  We don't want to actually seal an earthen plaster such as using boiled linseed oil over it.  We want to allow the earthen plaster to breathe and hence the reason for wanting to coat with lime plaster.  We do not want to use any cement or portland cement, at all.

I have read (somewhere) that you can add lime to earthen plaster, but details of if dry lime or lime putty were not given nor the ratios, so I don't know if this is an option or not.  Can you really mix lime, dry or putty in with the clay, sand, straw plaster, and then use straight lime plaster for final two coats, without having 4" thickness of earth plaster?  I like the idea of using only the earthen plaster, but I don't want to have to maintain it every year and thought with the lime plaster over it would be less maintenance issue, with maybe lime wash every few years or so, but now after reading the one posting, I am afraid to do this as I do not want the plaster to fail.

These are the two websites that concerned me with differing opinions:  

http://buildnaturally.blogspot.com/2013/10/lime-over-claywhen-is-that-ok.html

(pages 7 and 8 of this pdf file):  file:///home/chronos/u-6b3dc8bb2c4a387df24972b04f63105b4eea20f0/Downloads/See%20How%20to%20Make%20Lime%20Plaster%20(1).pdf

Anyone with experience with clay plaster for beginning coats of render and final coats of render with lime plaster, your thoughts will be greatly appreciated.

Thank you.

M.A. Carey

I do have experience with exterior earthen plaster and exterior lime plaster.  I have a cob house with an attached strawbale walk-in cooler for our farm.  The strawbale cooler has cob infill, then earthen plaster, and then lime plaster.  It has been mostly done for 2 years, and even though there are some areas that still have only one coat of lime plaster, they are wearing very well.  I can see no evidence that the earthen plaster substrate is stressing the lime plaster.  The base coat of lime plaster was supposed to be covered with a finish coat all over, but is still only half done, as it has been so patiently showing no wear, that it keeps falling down the priority list.  However, I will say that the wall that has the second coat of lime plaster looks so beautiful that I do intend to add the second coat everywhere.  Then, the general maintenance plan is to "whitewash" it with thin lime solution every few years.  Earthen plaster is generally more forgiving and easier clean-up, but the weatherability of lime was important to me over the strawbale, as I have  fear of moisture seeping in during driving rain.  I do love plastering, and am now in the process of plastering over my outdoor cob oven.  With a good amount of screened horse manure, the earthen finish plaster seems to wear as well as lime.  One can always oil it for even more durability.  Then it is basically the same as the finish coat of my earthen floor, which baffles me with its durability.  However, the lime plaster is literally rock when fully cured in moist conditions, so is even MORE durable than well-prepared earthen/manure plaster.

Laura,

Could you create some update on the performance of your earth/lime hybrid plaster after four years?
In spring I'm going to plaster our house. It's built from compressed earth blocks (stabilized with cement).
Interior I'm going to plaster with the clay mix of similar composition as the blocks (going from richer to leaner for each layer) and then limewash. For the exterior I need something more resistant to elements (rains 12-20 inches here in wet season). I like your approach of using earthen plaster as the base coat for lime based one. I have read many articles that such a combination is likely to fail.

For what it's worth, I have also heard that lime plaster over earth plaster is not a good plan. I heard that if you use a lime/clay mix for your base then a lime plaster finish will not have the same problem. But I have also heard that you need enough lime in the lime/clay mix -- like 20% lime; it can't just be throwing a little lime in.

I also am only at the booklearning stage and have just started plastering, so I am no expert and don't know the "whys" behind the above guidance. I did take a couple plaster workshops with SW Solar Adobe and with the Steen family / Canelo project.

If it were me in your situation I would use a "lime-stabilized earth plaster" recipe for the base.

We are using lime plaster but so far it's been on scoria earthbags and we will be using lime stucco over a stick built wall.

Thank you Kim for your input.

I always wonder how the line would stabilize clay. Would it make more resistant to water penetration and lime is already letting water through?
Cement in clay on the other hand definitely makes it water resistant. When we were building our house (I worked as a tender), sometimes I would crush the cement stabilized clay mortar from the previous day, add water and make it into new mortar for filling reinforcing holes (these without rebar) in the blocks. After drying it was still repelling water, despite the fact that the cement was already bonded to water the previous day. So it really works.

I think that it's possible that for some people lime over clay mortar works, because they are using some lime-compatible clay that creates some lime-clay union despite having different physical properties.
Last year we plastered the chimney when laying roof tiles. I used remnants of the CEBs that I pulverized in the crusher, added some cement and clay to the first coat, scratched it and the second coat had less cement. The plaster is still there, despite the rainy season and freezing temperatures I experienced. I think I will use the same mix for plastering entire house. I have limited amount of CEB remnants so the rest will be just the clay mix I used for laying bricks.

Three years ago I did an experiment - plastered a block with 3:1 lime plaster, but after rains it completely separated. Permeability of the lime plaster allowed water penetration and loosening of the underlying surface of the block (they become slightly slippery after rain)  which resulted in separation.

I was also thinking about using some home made hydraulic lime, by mixing high calcium hydrated lime with some pozzolanic agent: brick powder, high-reactivity metakaolin (HRM), but these substances would be too hard to obtain or expensive to transport in large amounts. On the other hand HRM is also constituent of portland cement so I suspect that just adding a tiny bit of white portland cement to lime would result in "hydraulic lime". I will experiment in spring - lime does not tolerate freezes.