I wanted to share a (relatively long) conversation with you all about lime plaster over an earthen plaster (clay) base that is happening now on a straw bale list serve. This conversation sums up what I have been seeing and hearing about quite a lot in recent years: failures of lime over clay plasters. I hope everyone considering this option reads this conversatrion and changes their mind. Here’s the letter from Chris, the man whose plaster has failed. My response is below.

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Hi folks,

As I mentioned in an earlier post, I’m building a small straw bale home in western Oregon. Since I’m largely inexperienced in straw bale methods, I’ve used the literature (with the help from a builder or two) to help inform my decisions on specific details, and for our upcoming bale plastering, my consultations have led me to seriously consider the application of an initial earth discovery coat with a lime finish on the external walls.

This method is widely touted by just about every bale book I’ve gotten my hands on, as well as in articles in The Last Straw However, I’ve come across a few cases (mostly in the archives of this listserv) of plaster failures at the earth/lime interface. It seems like these have been the result of one or two factors: improper keying of the lime coat to the earth discovery coat, and differential swelling of the clay against the lime when the wall gets heavily saturated.

All this thinking about external wall renders has led me to wonder why there’s this disconnect between what the books/articles recommend and a the experiences of a few unfortunate folks. Are failures rare enough to still warrant the application of lime over clay? Or, as more buildings stand up to the test of time, will this technique be less enthusiastically endorsed? I’m curious to know what folks think.

We’re one of those failures. We live in a very wind-blown, cold wet in the winter location. After doing all our research, talking with experts, etc we decided on earthen covered by lime. Seemed ideal for a SB in our area. Since we have access to an entire dry lake made of clay this was perfect. We did multiple batches and put the samples on bales. Left them in the elements. Recorded our results. Chose the best. Were meticulous about our formula. AND all the walls done this way failed. We finally removed it all off two long protected walls and put up just lime. We have two more short walls to remove and redo with lime. It’s HARD when you think you are finally done with your house to go in and rip it apart again!

What we discovered:

Earthen plaster doesn’t stick to chicken wire over wood.

We have one hybrid 2×6 wall. In some places the plasters stuck WELL and when we started removing it the lime pulled huge chunks of earthen with it. In others- no bond at all. Not sure why. They were scored the same, same formulas, and in some cases were immediately adjacent to each other. From our observations of the bonded sections they seemed to have somewhat more fiber (we used Pre Processed Straw- horse manure!). Perhaps the manure had different PH? consistency? variations on fiber?  The PH of the clay might be very important as alkaline tends to more salt and that seems to interfere with bonding

Lime is easier.

No one might say lime is easier to work with but we like it. We like working with it and we like the way it functions after drying. Of course it took me quite a few failed attempts to LEARN lime!! We had HIGH hopes for earthen covered by lime but now wish we’d never done it. IF we had to do it again we would try to standardize EVERYTHING. We would test the clay and we would test each batch of manure (or just use straw and test it!) AND each final batch of plaster. The varirables in earthen just seem to be quite broad and not as easily controlled as using lime plaster.

For those of you SB experts- please dont’ flame me. I KNOW there are probably many things we did wrong- but just like most owner/builders and most SB builders, thats the name of the game. You shouldn’t have an entire system fail when you are attempting to do everything right after research. THAT learning curve is too expensive. This is our experience, and simply our opinion. Perhaps it will help someone else avoid the pitfalls we fell into and discover a way to make earthen/lime work well!!

-Chris

Here is my response to Chris and the group:

Hi Chris. First of all, I’m very sorry to hear of your plaster failure. That must be very disappointing and emotionally taxing.

I have seen and/or heard of a lot of failures of lime over clay plaster. I have been recommending, for years, that people not do it. There are different reasons for the failures, but they all come back to the fact that there are two different materials that simply don’t easily work well with each other. Like you said, it is a very steep and expensive learning curve. I’m sure there are places where this has worked that we don’t hear about, or perhaps those are destined towards failure in the future as well, who knows.
I suggest to people that they either do an earthen plaster through and through and through or they use a lime plaster. Don’t mix and match is my opinion. I am a big fan of Natural Hydraulic Lime plaster, although it is expensive. Is it more expensive than plastering a house twice because of a failure? I don’t think so,. I believe that the emotional expense of such a failure is higher than the financial expense of the NHL plaster.
To me, especially in wet areas, lime is the way to go for exterior plaster. It is durable, relatively easy and fun to work with, and time tested. I hope that your experience with lime is perfect and this is the last time you plaster your house! :)
Happy Baling!
Andrew

About the Author

Andrew Morison is a specialist in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs, as well as his hands on workshops. You can check these out at www.StrawBale.com/store.

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