We constructed the outer walls for the garden shed using two different techniques — light straw clay and chip-slip (wood chips mixed with clay slip), in order to compare the two techniques in terms of speed and ease of construction, as well as insulative properties.
Our instructor this week for natural wall systems was Erica Ann Bush, a natural design-builder. You can check out her work on her website Day One Design.
We started off the week with a demo on straw bale construction although we didn’t actually build any straw bale walls for the shed. In the straw bale demo, we set up a mini wall by the workshop, with internal pins of bamboo stakes. We then compared that technique with making an external wire frame for the straw bale wall.
Anyway, as I mentioned our shed didn’thave any straw bale walls. Instead, we constructed the walls out of light clay straw and chip-slip.
Light clay straw is – as its name implies – a combination of straw and clay slip (clay that has been diluted with water.) The clay content in the mixture is relatively low – ideally the straw pieces are lightly and evenly coated with the liquidy clay slip. Light clay straw can be mixed by hand, though for larger jobs there are some designs out there for machine mixers (see pic below of Preston the Intestine.)
Forms are constructed on the inner and outer faces of the wall, using plywood or other material, and the light claw stray is deposited into the forms and tamped down, in layers of about 8 inches. Every four feet or so of vertical rise (at the junction where the horizontal trusses bridge the studs) long pieces of bamboo are laid horizontally into the clay straw to provide extra cohesion for the wall. When the clay straw is packed up the top of a form, the form can be moved to the next higher section. When the wall is totally packed, all for as are removed and the wall is allowed to dry – at a rate of approximately 1 week per inch. Our wall is 7″ thick.
Chip-slip is another material for walls. It is made from wood chips that are coated with a clay slip. You want to use wood chips that are free of bark, because insects are attracted to bark. Instead of plywood forms, the chip-slip walls are built with wood lath forms, small pieces of wood horizontally attached to the wall studs, with spaces in between them. The chip and clay slip are mixed in a tarp and dumped by the bucketful into the wood lath forms. This system is faster to build than light clay straw walls, but it’s a relatively new technique and its insulative capacity and long term performance are not well established yet.
There are good sources of clay on the Aprovecho campus, so we harvested the clay for the walls here.
Plywood forms for the light clay straw wall. After you pack a portion of the wall, you can remove the forms.
We also spent an afternoon making cob to repair a wall on campus and also made some adobe bricks. We didn’t have any particular plans for using the adobe bricks, but they are useful to have on hand for building projects. Adobe bricks are composed of the same material as cob (clay, sand, straw), just made in a slightly wetter mix so you can pour them in brick form. We mixed the materials by foot and then filled small and large wooden forms to make different size bricks. It’s best to moisten the forms first so the bricks don’t stick. After filling up the forms, remove them gently, and allow the bricks to dry in the sun, turning them on occasion.