Straw as a building material may seem sketchy the first time you think about it. But the more you learn, the more it makes sense. I am in the middle of a 6-day workshop adding the straw bale component led by straw bale building master and local legend Mark Morgan of Bearpaw Design and Construction . We are working on a structure designed and built by Roald Gundersen Whole Trees Architecture and Design ( recently featured in the New York Times).

...Construction site.

One of the things I am loving about straw bale is that there is so much craftsmanship involved in building well with it, but an enthusiastic beginner can jump right into the process. Especially when being guided by someone who knows the material well, as Mark does. It is invaluable to have a builder who is familiar with sustainable materials.

If you are going to build green, using materials that are renewable and locally available, you have to be able to respond to unique challenges every step of the way. This building includes two circular rooms. Roald’s design was inspired by a discarded hub from a semi-truck wheel, which he found at another building site. It became the joining element to connect the roof, which creates an elegant structure. That meant bending rectangular straw bales around its curving walls.

Curved walls are readily done with straw bale. We built an impromptu bale compressor on the work site that can be used for compressing bales to resize them or reshape them.

...bale compressor

For the round walls, each bale needed to be compresses on one side and retied tighter on that side. That caused the exterior side of the bale to fan out slightly.

Before being placed, each bale was tapped perpendicularly over another horizontal bale to increase the curve a little more. Then set in place.

There is a limit to how much bales can curve. The minimum dimensions you can use this technique for are rooms with an outer diameter of 18 feet.

As each row is added, all the chinks created by the irregularities of the structure and the bales are filled with a mixture of loose straw coated in a clay slurry. This is called light straw clay .

In some cases, builders can use clay dug from the building site, but the soil here was very sandy, so we used a low-fire red clay from Continental Clay .  Whatever the clay source, it is mixed with water to the consistency of cream.  (We tossed a few fist-sized, sharp-edged rocks we found at the site into the mixer to help break up the clay.  They serve the same purpose as bee bees in an aerosol can and are removed before using the slurry.)

We made a mixing container of straw bales lined with a tarp, filled it with loose straw, tossed in dollops of clay slurry and mixed it with our hands until every strand of straw was lightly coated. Earlier this summer I attended a workshop using this straw clay as an infill to insulate an entire building. In that case, because we needed much more, we mixed it on a 10×10 foot flat surface with pitch forks by tossing it like a salad. The pitch fork method is a little easier for large batches, but I liked getting my hands into the mixture and really feeling how much was enough. You don’t want it sloppy wet– just barely damp, but each straw needs to be coated evenly. When you are done, clay rinses off your hands easily with water.

Mark warned that the chinks should not be filled too tightly. Compressing the straw can reduce its insulation value. And the insulation value for a straw bale structure is R40.

As the walls went up, I could feel that R value intuitively. The sense of shelter that you get inside a straw bale structure, from the very beginning of the building process is palpable.

My next post will be on stitching the walls together. This is what gives a straw bale wall great strength.