The second weekend of our straw bale construction workshop (led by Mark Morgan of Bearpaw Design and Construction ), we got a chance to try earthen clay plastering on the interior walls of the building.
The first weekend, we troweled concrete stucco on the outside of the straw bale walls. I found it an intimidating process. The Portland cement, once mixed with water is on a one-way trip to the solid state. A chemical process is initiated, and you have a limited time to apply your mix. It is also a toxic substance that you are trying (in my case very unsuccessfully) to keep off your skin.
Also, concrete that slips (oh, so easily) off your trowel if you don’t have precisely the right trajectory and propulsion as you approach the wall hits the ground and is not reusable. An amateur can be very wasteful in applying concrete.
The inside walls of earthen clay were a different story!
I love working with this material. The clay is nonthreatening and very forgiving.
I gradually mastered how to hold my hawk of clay in my left hand and scoop up trowelfulls with my right hand and move the trowel surely enough to make contact between my dollop of clay and the straw bale of choice.
But I’m a long way from a master plasterer at this point, and I had plenty of trowelfulls of clay dropping to the concrete floor just before meeting with the wall. No problem! Just scoop it up and try again. That’s what I call an agreeable material!
The clay needs to be worked deeply into the ends of the straw and cover it completely. Eventually three layers of clay will be about 2” deep, but each individual layer is very workable.
Mark does not build his straw bale houses with vapor barriers. It is perfectly alright for the walls to absorb moisture from the air. In fact, it’s preferable. In humid weather your walls act as a dehumidifier, drawing the moisture out of the air and into the wall. When the conditions become dry, the walls release their moisture load back into the air, making it more comfortable in both conditions. Mark assured us this does not damage the wall in the least. I’m sold.
Now I find that plastering the walls is one of the tasks I’m most looking forward to when we build.
It’s easy to see how the creation story in Genesis depicted man being formed from clay. When you have your hands in clay, you feel like anything is possible.
There is a very good book out by Cedar Rose Guelberth and Dan Chiras called The Natural Plaster Book: Earth, Lime and Gypsum Plasters for Natural Homes. 
Some of the benefits that they list for earthen plasters include:
Add Unrivaled Beauty to a Home
I can attest to that. Just the first coat that we applied made the room incredibly inviting. Clays come in many colors creating endless options. Some people add mica flakes to give a wall a bit of sparkle. Others rub oil and beeswax on earthen finished interiors walls for added luster. I’m really looking forward to exploring this possibilities.
Create a Soft and Quiet Surface
Even after just the first layer, I could sense this quality in our clay-plastered room. Clay has a sensuous feel. It is so elemental and timeless and just interacts with its surroundings in a gracious and welcoming way. And the rounded corners!
Fun to Work With
I’m here to tell you that they are. That is not to say that they are easy to work with. I expect I will have to build up my upper body strength to tackle plastering and I expect to come out the other end with very buff arms indeed.
Safe to Work With
After the stress of cement plastering, I loved the non-caustic quality of clay. In fact, this is the very stuff that people pay big money to have rubbed on them at some spas to make their skin glow.
Allows Work to Proceed at a More Relaxed Pace
I found this to be true. The clock was not ticking and the whole process became timeless. It was almost like meditation. And if you have clay left at the end of the day – no problem. Just cover it and continue tomorrow.
Easy to Clean up After
I was amazed how easily my hands rinsed off in the tub of increasingly brown water sitting outside. Wet clay just melts away in water from both hands and tools.
The nice thing about plastering over straw bale is that the whole process is a bit to begin with and meant to have irregularities. They add interest. And they put the process within the power of an ordinary person who wants to make their own walls.
Categories: Eco architecture
Digging in the Driftless 
Sounds fascinating. Obviously you are enjoying this whole learning process. I’ ve heard of straw bale construction but don’t know anything about it. Nice to follow your first-person reports. It does sound inviting.
So you are planning to use this process when you build your house? Or what? I think I missed that somewhere.
Hi Dennis,
It is indeed a very fun learning process. I fell in love with the concept of straw bale the first time I learned about it and saw those deep-set window wells that such thick walls create. They reminded me of many buildings I loved when I lived in the Netherlands. All of them very old, and still standing.
Learning the hands-on details is just increasing my love of this material. It’s environmentally friendly, sturdy, durable, beautiful, and just plain satisfying to work with. It’s like building blocks and mud pies for grown ups.
Looks awesome but I have to ask, why would you put concrete on the exterior? the point of lime and earthen plasters on strawbale and cob homes is to allow the wall to breathe, concrete does not allow this and will promote rot with the slightest introduction of moisture. Thank you and I look forward to your reply!!!
David
New Jura Natural Building
Thanks for responding, Dude. You’ve raised a very good question, that I am going to have to ask Mark Morgan who led the workshop. That very concern did cross my mind, but when one is taking in so much new material at once, some things do slip past.
I didn’t actually like working with cement. It was by far the least natural material and process of the whole workshop.
What would you put on the outside?
Denise
I do a forced spray of a clay slip on the bale then allow it to dry and then tool anything sticking up high, then a earthen (cob) scratch coat and the a lime or earthen plaster, with the earthen plaster there will be more up keep due to it not being water proof but if you did a lime plaster with a color pigment over it then it will last years and allow the wall to breathe!!!!
David
Hi David,
Thanks for your prompt and very specific response. I spent some time exploring your website, and see you have got some great projects there.
I also noticed that you live in Texas, which I think of as a much drier place than Wisconsin. We get a series of torrential rains through the summer. And the best projections of the state’s climate change team is telling us to be ready for more of this. Including less snow and more freezing rain in the winter.
Do you think your exterior surface is up to that kind of beating?
Doug and I are still in the info gathering mode and weighing all the options.
To me green also means durable, so it won’t have to be redone for a long, long time.
I like what you are describing and want to have truly breathable walls.
Denise
The key to massive water amounts is adequate overhangs of the roof structure, here in Texas we also have torrential rains storms, just not as many as you. Every roof system I do I add overhangs (eves) of 4′ or greater if allowed to minimize the amount of splash or direct impact. Then rainwater gutter systems or rain chains, also gravel beds with French drains at the eve ledge (this is the spot rain drips from the roof and hits the ground) to defuse as much splash as possible and to quickly move the water away from the base of the structure. It is a fact that longevity is very green but at what expense? If the concrete cause rot and deterioration of the bale wall then how green is that? I hope I am not sounding like I am scolding you about the choice, I just want your home to survive as long as possible!!
I certainly don’t take it as scolding.
Choosing to build using non-standard methods was an easy decision. So many aspects of current building practice are wasteful and unsustainable.
There is so much to learn — or rather relearn.
My eyes were first opened when I lived in a 300-year-old farmer’s cottage with a thatched roof in the Netherlands. I know there are building techniques that really stand the test of time, and feel better to be in than our contemporary structures.
Thanks again for your feedback. I’ll be following your website.
Denise