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The Eartha Madre Project

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Everything is ready to start the in-fill

It was definitely a labor of love……….with the emphasis on labor!  We chose to use straw clay infill or “light clay straw” because straw was readily available locally, we had plenty of clay in piles around the building site from the excavation of the  foundation and we were impressed with the fact that in a "typical" 12 inch straw clay wall like the ones we have here in our building, using this material gives us about an R-40 straw clay insulation factor. 

All the materials prepped and ready to mix

We hosted two separate straw-clay workshops as our construction progressed and were fortunate to be able to use a special mixing machine to mix the straw and clay for much of our straw clay work.   We used the leaning wire mesh box to sift our clay before mixing it with the water to make the slip.  Normally the process for mixing the straw and clay is even more labor intensive.  The traditional method of mixing the straw and clay requires first mixing the clay with water making up a slurry with the consistency of cream or a thin milk shake. The next step is spreading the straw out on the ground and spraying it lightly with water followed by the slurry and then toss and mix the straw so that it becomes lightly coated.   We used rakes and pitchforks to toss the mixture when we had a second workshop without having the benefit of the special mixing machine that we used during our first workshop.

A straw clay

“production line”

All the workshop participants got an opportunity to be involved with every step in the straw clay mixing process.  The straw was broken out of the bales and tossed into the mixer followed by regular light spaying with the hose and regular additions of the liquid clay slip.  The other end of the mixer was equipped with a wheel barrow to catch the mixture and transport the wet mixture to the walls, where it was packed into the wall forms and tamped.

The clay slip coated straw ready for packing into the wall forms

Having the use of the straw clay mixer machine really accelerated our wall filling process.  In keeping with our “healthy home” approach we wanted an insulation material that was non-toxic, renewable, and locally available.  Low cost straw makes excellent non-toxic insulation and our research led us to clay-coated straw with all its inherent benefits.   A light coating of clay acts as both a binder and preservative and as the clay dries, it binds the straw together in a surprisingly rigid mass.   It's a "natural styrofoam".   Clay coated straw has been shown to last over 700 years as a non-deteriorating insulation and it has an R factor of about 3+ per inch.

To prepare for filling the walls we screwed two plywood boards temporarily to the posts and around door and window frames, one on each side.  Then we filled the resulting cavity with straw-clay and tamped it down. We started by using 2x4s for the tamping but ultimately ended up stomping it down with our feet.  The two side boards were immediately moved up the wall and stuffed again and again until the wall was completely filled.  To prevent to much settling of the straw clay as it dried, we installed bamboo poles horizontally between the posts and framing as we worked our way up the wall.

The end photos from two great workshops

Although we still had two other wings of the building to build and 6 other walls to fill with straw clay, it was amazing seeing the timber frame, covered with a good “hat” and the completed straw clay walls (above left).  We ended up having one more straw clay workshop at the site to complete the other walls (above right).

Fully dried straw clay walls ready for plastering

The straw clay walls went through several stages during their drying process.  At one point about 45 days after the walls were stuffed, we had a few sprouts growing out of the walls.  When the walls were finally completely dry, we covered them with chicken wire (to meet codes) and they were ready for plaster.  We were asked on more than one occasion at this stage of our construction if the place was a new barn!

Our clay “seasoned” in in the weather for 18 months waiting to be used for plaster.