Friday, September 27, 2013

Building a Straw Bale Home: the Cal Cases Community

Cal Cases is an intentional community of around 30 people (including children), located near Santa Maria d'Oló in the foothills of the Catalan Pyrenees. In 2010, I had the good fortune of working with Cal Cases and I must say that it was a tremendous experience, living and working with such a happy, successful community! It was a really positive experience for me to get to know these people and to learn about their work together. Its members come from all different walks of life, brought together by the common desire to live sustainably, in harmony with their values and with the world. The result is a giant, happy extended family that has impressed me greatly through their compassion, generosity, idealism and resourcefulness.

Not to mention that central Catalunya is an extremely beautiful region: rugged, mountainous, full of forested wilderness, with small medieval villages scattered throughout. From the ridge where Cal Cases is located, you can look out across the plains to the south and see the holy mountain of Montserrat far off in the distance.

But I digress! In 2010 I worked with Cal Cases in the role of master carpenter in the construction of a large timber frame, straw bale house. It would be divided into 4 apartments, to house the remaining members of the community who were still without a home of their own on the property. It was a very large project that would span 2 years, involve scores of volunteers, neighbors and friends, and result in so many beautiful photographs that it's going to take me 2 blog posts to document the process!

One feature of this project that I very much enjoyed was the importance we placed on using natural materials, and on salvaging what we could when nature couldn't provide. Because of everyone's dedication to this ideal, the overall cost and carbon footprint of the building were kept extremely low. And we were in fact able to achieve standards of construction, insulation, low energy consumption, indoor air quality and beauty that far surpass the norm these days!

Now onto the details. The project incorporates many elements of natural building, such as timber framing with local round timbers, a giant earth-sheltered super adobe wall, straw bale walls, a straw bale insulated green roof, using glass bottles to make beautiful artistic windows, earthen plastering and natural paints, etc, etc. I'll do my best to describe each technique as we come to them chronologically through the build.

The building was designed to incorporate the structure of an old greenhouse made of telephone poles that had been built by the previous owners. It was located on a south-facing slope with a steep incline behind it, a short walk from the gardens and communal spaces of the Cal Cases community.

With the help of neighbors and friends, the hillside was dug out to accommodate an earth-sheltered north wall. Possibly the most crucial element (and the least talked about!) of a successful earth-sheltered building is a robust drainage system: Water from the hillside behind the building must be diverted with a large drainage system located behind the earth-sheltered wall, and carried away from the building with French drains. Failure to properly drain the hillside would result in a large amount of water coming into contact with the earth-sheltered wall, leading to water and air quality problems inside the building. I'll write more about the drainage system as we go along.

Trenches for perimeter french drains, utilities, and for the foundation were dug at this point, and walls that were going to be built with straw bale were given stem walls. The stem wall is an important feature of straw bale construction: it keeps the bales raised up off the ground to protect them from overexposure to moisture (puddles, splashing raindrops, snow mounds, etc). We used limecrete instead of concrete to reduce our carbon footprint, and to avoid supporting the notorious cement industry.

The timber frame of the building was completed using pines felled in our neighbors' forest. Cal Cases arranged this by helping to shear sheep with the neighbors, in exchange for some trees when the building season arrived. This is exactly the kind of resourcefulness that impressed me so much with Cal Cases. Not only did acts like this save money in the long run, but they also helped improve the neighborhood economy and neighborhood relations. They managed to do quite a bit of trading for materials and services rather than dealing with money all the time.

The next month or so of the project was spent building the massive super-adobe earth-sheltered north wall. By giving this wall a curve, we strengthened it against the weight of the hillside behind it. 

The first few layers of sacks were filled with gravel and installed beneath ground level as the foundation, tamping each layer as we went. Barb wire was placed between the sacks to prevent them from slipping out of place. We continued filling the sacks with gravel until we were about half a meter above ground level, to serve as the stem wall. The gravel serves as a capillary break that prevents moisture in the ground from wicking up into the walls. Then a vapor barrier of polypropylene sheeting was added over the stem wall, to be sure no moisture finds its way into the earth-filled sacks above.

We then filled each layer of sacking with slightly moistened, clay-rich subsoil and tamped them in place. Instead of the traditional barb wire, we used bramble thorns between these layers of sacking as moisture was no longer a threat to organic material. The sacks were strapped to the timber frame as we built. This was intended to help unite the different building mediums and prevent them from shifting apart. Wooden strip anchors or “deadmen” can also be seen sticking out from between the sacks. These are pieces of wood with nails sticking out of them, to help them grip the sacks. These create points where wooden studs can be screwed in place later on.

Finally, the drainage system behind the super-adobe wall was completed. First a layer of geotextile was attached to the cliff face, behind the wall. Geotextile serves as a filter to prevent sediment from slowly clogging the drainage systen. Then a waterproof cavity membrane was attached to the back of the superadobe wall, forming an impermeable barrier that facilitates the downward drainage of water to the French drain installed below. Finally, the space between the geotextile and the waterproof membrane was filled with rocks and gravel, the geotextile was folded over the top, and topsoil was added on top of that.

The super-adobe wall was to be rendered with earthen plasters, so the first step was to apply a thin clay slip to the sacking. Our subsoil was very clay-rich, so we sifted it very fine and added about 2 parts water to every 1 part sifted subsoil, to form the clay slip. This mix was applied using a converted hot water tank, pressurized with an air compressor. This is a common method of applying clay slip to large-scale projects that saves a lot of time and also ensures a good, even, penetrating coat.

A coarse earthen mix with long straw was then used to patch the cracks between the levels of sacking. This was sufficient preparation for the application of the brown coat, an earthen plaster base coat consisting of clay, sand and chopped straw.

We finished building the earth-sheltered, super-adobe north wall just in time for the spelt harvest, which provided us with the straw bales necessary for the remaining walls of the house. Cal Cases, once again demonstrating the love and excellent planning that had gone into this project, had agreed for the neighbors to grow a crop of organic spelt on the property. In exchange, we took a small amount of the grain for our horse El Duque and baled the straw for use in construction.

In the photo above you can see the classic method of safely stacking the straw bales for construction: first a layer of pallets goes down; then the first couple of layers of bales flare outwards, before sloping back in to form a pyramid. This makes sure the water can run down off of the tarps and drip down to the ground, without getting the bales wet. It's very important that the bales are protected from moisture, starting even before the harvest comes in. Not until the bales are stacked, the roof goes on, and the walls are plastered are they truly safe from water damage.

In these photos you can see the stem walls going in that butt against the earth-sheltered super-adobe wall. Plastic piping was installed beneath the stem wall because later we will be compressing the straw bale walls. The piping allows us to pass straps under the wall for compression. Afterwards these pipes will be filled in and covered.

Now the walls were ready for framing, before the bales go in. Polypropylene vapor barriers were installed on top of the stem walls, followed by OSB wooden bond beams. Doors and windows were framed, and we also installed studs to reinforce the old telephone pole structure built years ago. These studs were installed 65cm apart, which was a little bit smaller than the straw bales we would be building with.

We used two different methods for baling the walls: in the southern section of the structure, which had studs, bales were squeezed into the spaces between the studs and stacked in vertical columns. These columns were then compressed with car jacks for 10-15% compression. This is a method advocated by Tom Rijven for using straw bales in conjunction with studs. The bales apply equal pressure to the studs on either side, strengthening the wooden structure.

In the northern section, there were no studs so we were free to stack the bales in a traditional running bond. Some hazelnut prunings from a friend provided long, straight poles for exterior pinning, helping to stabilize the walls. Wooden bond beams were added and the wall was compressed using trucker straps (passed through the piping we had previously installed in the stem walls). We made sure the bale walls were compressed 10-15% before fixing the bond beams in place. They are not visible in the photos, but cripple studs were installed on top of the bond beams to securely fasten them in place.

Meanwhile, the rafters and ceilings were going in, so we could get the bale walls roofed as quickly as possible. We planed our round timbers for use as rafters in the north-facing roof, to save ourselves time later on when installing the ceiling planks.

I threw in a photo above of the shearing that had gone on months ago, before construction began. Now was the time to finally make use of this wool, which our neighbors had gifted to us for helping out with the shearing. Wool makes a great ceiling insulation, but needs to be treated to protect it from pests. We heated up water and added borax salt, which was then applied to the wool and left to dry. The salt serves as a deterrent to pests.

The south-facing roof was to be covered with willow twig fencing, giving the appearance of thatch over a bituminous membrane. So over the ceiling went building paper and the wool was used to fill the gaps between rafters. We were careful to leave an inch or two of air space between the wool and the decking to provide air circulation and prevent moisture buildup. On top of the decking went some felt to protect our expensive and fragile bituminous membrane, which you can see being installed above. The willow twig fencing was later fixed on top of wooden battens - carefully installed without puncturing the bitumen. 

Once the south-facing roof was watertight, we turned our attention to the north-facing roof, which was to be a straw bale insulated green roof. You can read more about that and the rest of this building project - and see the photos of the finished building! - in Building a Straw Bale Home: Cal Cases (Part 2).

- Ben

Next post: Building a Straw Bale Home: Cal Cases (Part 2)
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1 comment:

  1. Qué bonita y detallada exposición. Gracias, Ben. "Molt bé" ;)
    Un abrazo desde Cal Cases.