Low-tech glory: Straw and clay
A lot of the attention in a sustainable building goes to the high tech equipment and mechanical systems. But at the heart of a project like Canada’s Greenest Home are some wonderfully simple, low tech and extremely effective structural systems like our clay plastered straw bale walls.
The north wall bales are installed and ready for plastering. The wall studs at 34 inches on centre are visible between the bales.
Installing the straw bale walls on the north side of our building and coating them in clay plaster is a strategy that combines low cost with high performance, and provides a window to a building system that is competitive with current energy-intensive practices but is also feasible in a world with a lot less fossil fuels to expend. These are materials that are locally accessible in most settled regions of the world, and the fact that one can base a very energy efficient home on them gives hope for a future when other materials may be much costlier or no longer available to us.
We installed our bales into a double frame wall system that mimics conventional frame walls, but with the studs placed at 34 inches on centre. In doing so, we create “bays” in the wall that are sized to the length of our straw bales, making bale stacking and plaster preparation very simple and straightforward. Unlike post and beam frames, no notching or cutting of bales is required, nor are heavy beams at the top of the wall. It is a very simple, very cost-effective manner to build a bale wall, and one that many professional bale builders find themselves gravitating toward.
Once the bales are installed, we use a “two-part, one-coat” clay plastering system. A thin coat of a wet clay plaster (1 part clay to 3 parts sand) is rubbed into the surface of the bales to provide a strong key into the straw and an adhesion layer for the bulk coat that follows immediately. This adhesion coat goes on very quickly. The bulk coat is a mix of clay, sand and chopped straw (1 part clay, 1.5 parts sand, 3 parts chopped straw). The more clay plastering we do, the more chopped straw we’ve added to our plasters. The bulk coat resembles a mix between cob and light-clay straw. This coat has enough tensile strength from the chopped straw to be applied to the wall at almost any thickness, from as thin as 1/2 inch to as much as 3 or 4 inches. This allows us to make a straight wall out of a lumpy, bumpy bale wall in a single coat.
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- All the ingredients are carefully arranged around the mixer to minimize effort for the mixer. Behind the electric mortar mixer is the bagged clay, chopped straw, sand and water.
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- The recipe for the plaster is placed on the sand pile where the mixer can refer to it when memory fails.
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- Water is the first ingredient added to the mixer. The quantity varies with the amount of water in the sand pile.
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- A masonry sand from a local quarry is added to the water. Having the sand in the mixer first helps break down the clay.
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- A bagged ball clay is added. While we often use local site soils, the bagged clays are handy when site soils won’t work.
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- There is a lot of chopped straw in a mix… in this case 9 buckets. It must be added at a pace that allows the mixer to churn it in without straining the motor.
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- The finished mixture is dumped into buckets to be carried into the house.
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- Lorna shows the sticky, straw-y finished plaster
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- We mix enough plaster to do an entire wall. If the plaster can sit covered for a few days, it starts to ferment which adds strength and water resistance to the plaster.
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- The north wall bales are installed and ready for plastering. The wall studs at 34 inches on centre are visible between the bales.
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- The wall is nearly finished being plastered. It has taken a matter of hours to apply the two-part, one-coat system entirely by hand. Hand plastering is much easier to master than using trowels.
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- The interior wall is ready for plastering, and the slip coat is starting to be applied. There are air fins at every seam to keep things air tight where plaster meets other materials.
We find that this type of clay plastering is a great deal more beginner-friendly than lime or cement based plasters. The clay plaster can be applied by hand, and no trowels or tools are required to make a very straight, even and beautiful wall. To achieve the same results with other plasters would take several more coats and a lot of troweling practice.
This part of the work is also very social, very engaging and a lot of fun. Building a house while up to one’s elbows in mud is a real joy. The fact that we are making an airtight, highly insulated and long-lasting wall system only matters after we wash our hands and look back at the beautiful walls!
