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Straw Bale Building: Adobe Plaster Interior Part 1

Dave L HeadshotAdobe clay is one of the most pleasant materials available for building. Adobe is forgiving. Make an error or have a problem and you can start over by wetting it down and peeling it off or retroweling. For newbies like us, this was really important.

Adobe clay is a natural material with only the embodied energy involved in transport, unless you count the energy expended by the builders. In many parts of the world, clay is abundant. For example, there are clay deposits in colors ranging from red to gray within just a few miles of the Bear Cave.  

Adobe clay is economical. Our 320-square-foot adobe cottage, the Bear Cave, has adobe block walls, adobe plaster, and adobe floors. Total cost for the clay was less than $500 in 2008.

        Straw Bale House Dining Room

When it was time to finish the walls on our straw bale house (above), we didn’t think twice. It had to be adobe. We used the same clay, with different screenings, for every application except the finish coat on the interior. For that, we bought some Kaolin clay mined and bagged in northern AZ and mixed it with 60 grit bagged sand. We love the outcome, a nice balance of finished wall with a flavor of rusticity.

        Chopping Straw

With the straw bales up, windows and doors installed, and interior build out finished, it was time to plaster. Materials for preparing the walls included adobe clay, chopped straw, and water. The first step was to chop straw. We had a few bales left from the house walls and bought a couple more from the local feed store.

At first, we chopped using a weed whacker and a thirty gallon heavy “rubber” garbage barrel. The straw chopped with this method was fine for filling seams, but when we began work with the trowels after this preparation process, we wanted more uniform chopping and borrowed a neighbor’s mulcher.  For some finish applications on the adobe floor, we ran the straw through the mulcher twice.

        Screening Adobe Clay

Our adobe clay, fresh off the truck, had been ostensibly screened to 3/8" minus. However, we had some variety of rocks up to 3/4" or larger. We set up two screens outside, one with 1/4" hardware cloth and one with 1/8" hardware cloth. For brown coat and scratch coat trowel work, we screened with 1/4" and for finer work used the 1/8".

     Filling Straw Bale Seams

For filling in the seams and holes left in stacking our straw bale walls, we mixed adobe clay screened through 1/4" mesh with chopped straw. The mixture was heavy in straw and the clay was simply a binder. Straw, clay, and water were put into our old cement mixer until the right consistency was achieved. Firm enough to hold when slapping it into the seams and wet enough to bond to the bale. For major holes and seams such as around window bucks and above the doors, we would push in as much as would stay, let it dry over night, and make one or even two more filling applications. This process was the first step in getting a straight and even wall.

     Screening Aliz

It must be noted that there are probably as many different ways to plaster a straw bale wall as there are plasterers. We have tried a few and this one worked well for us. There are many who will swear that there is only one method - theirs - that should be used. Probably not! This one worked for us and I have seen other, different, processes that turned out beautiful walls as well.Many right paths to a desirable result.

One of the methods often promoted is the use of poultry net or chicken wire as well as metal lath and other such materials as the base for plaster. I have to come right out and say that I hate, yes – hate, plastering with adobe over chicken wire. Granted, it grabs the mud, but often leaves air space between straw and plaster that compromises the integrity of the wall. I know that if other plasterers read this, many will strongly disagree. So it goes.

I do agree that it is important to provide a surface with “tooth” that will allow the brown coat to adhere to the straw. To that end, we did a final screening of our adobe through a kitchen colander, getting a very fine and uniform clay.

     Slip Coat Mixing

Mixing water and this fine adobe gave us an aliz or fine plaster about the consistency of heavy paint.

     Spraying Slip Coat

With a sprayer designed for joint compound purchased from our hardware store, we sprayed all the surfaces we intended to plaster. Please note, there will be over spray, so mask all the windows and doors as well as any outlets or fixtures already installed that you don’t want coated in a fine layer of adobe clay. As we wanted uniformity on our walls and out interior build out was done with frame and sheet rock, we taped the sheet rock and then sprayed it with the aliz as well.

We did not put either a brown coat or a scratch coat over the sheet rock and the finish coat of plaster adhered beautifully.

        Wall Prepared for Plaster

This wall is now ready for the trowel!!! 

The pictures you see here were taken while preparing the interior walls for plaster. The exterior wall preparation was identical. The finish plastering process was different and will be addressed in a later blog.

We have been through two monsoon seasons and one winter with our new home protected by adobe plaster. It is withstanding temperature swings over 100 degrees, driving rain and snow, and the fierce Arizona sun. No problems.

Plastering is at once the most labor intensive and the most satisfying activity in building a straw bale home.  If you are going to create a straw bale home with adobe plaster, I suggest reading about the process. I also suggest a practice wall. Stack and support three straw bales, attach a layer of burlap on which to apply plaster, and apply different plaster/straw mixes until you find one that works for you. After you check out one formula, just peel off the burlap, rinse it off, and start over  It would be pointless for me to try to give portions of straw to clay as all clays are different. Besides, it's fun to experiment.

For more information on this process and for other resources and books, I hope you will visit me at I would be happy to answer specific questions here or on my web site.

So, read, think, plan, experiment, and, above all, enjoy the process. It’s worth it!