Straw Bale Build: Foundation

On our days off from the caravan rebuild, we’ve been helping a local French man Philippe who is building a straw bale house. He built his garage using the same technique last year as a test. This year, he’ll be building his house of about 45m squared 5.5 x 8, a little bigger than what I have in mind for myself but still very similar in scale.

For those who may not be familiar, to build a load bearing straw bale house like the one above, you stack straw bales up like bricks, plonk a roof on and coat the outside with mud plaster. It is incredible because

  1. It’s easy. Anyone can do it.
  2. It’s fast. One person can build one in a summer.
  3. It’s cheap. Straw is a waste product of grain production. Mud is from your land.
  4. It’s biodegradable. At the end of its 200-300 plus years of life, chuck it in a compost.
  5. It’s insulation capability is superb. Regulates humidity and keeps your house comfortable.

If I was a politician, I’d put a law into place where a large percentage of new builds must be done with straw. The more I study it, the more I’m convinced that it really is the perfect building material (for this region of the world at least).

When we arrived, he had already dug the trench in the foundation and filled it with gravel to let any water flow out. On top of the gravel, he had layered 3 gravel bags and 2 earth bags pounded into shape and strapped in that order. This is to eliminate any chances of water from moving upwards through capillary action.

We popped in to help create a bracket, levelling it perfectly as we go. We then filled this with lime (a traditional cement alternative with 1 part lime/3 parts sand). I’ve read a lot on lime but it was the first time I’ve had the opportunity to work with this material and it was surprisingly dry compared to cement – almost pushing it into shape. The surface felt dry and brittle the next day but lime slowly hardens and cures over the next two to three weeks. He’d covered the entire thing with burlap sacks, old sheets and cardboard boxes soaked in water to slow the drying process.

This week, we filled back the earth around the exterier perimeter and added gravel. A mud plaster, made of 4 part soil, 4 part sand, 2 finely chopped straw and half a litre of wheat flour water (1 flour to 9 water), was applied to the foundation earth bags. This was to protect the polypropylene bags from degrading under the UV light.

The soil, which he dug out for the foundation earlier, is really silty with a tiny amount of clay so he uses equal amount of soil and sand but if the soil is heavy in clay, the percentage of sand must go up to stop it from cracking. He recommends testing the mix first, making sure it sticks to something fairly smooth and dry it to see if it starts to crack.

He’s also showed us an experiement that he’s been conducting on using adobe earth bricks. A mixture of 6 part soil, 1.5 sand, 1.5 straw, 1 water filled into a brick shape and then dried for a couple of weeks. No firing, no binding chemical, just plain soil mixed and dried. The picture shows a screw in the brick holding 50kg. I knew houses in drier climates were made of soil bricks but I’ve always doubted it’s strength until I saw this.

I’ve been really struggling to find the right material for use as mass within the house. I even considered using stone – strawbale on the outside, stone on the inside – probably very effective at temperature regulation but not very practical with a meter thick wall. Adobe bricks seems like an easy, cheap, environmentally sound, flexible and adaptable material to build walls and add heat sink mass.

One of the exciting things I’ve learnt working here is that the battery powered tools have come a long way. Philippe uses a set of hand tools including a round saw, an angle grinder, two drills and a multitool by Milwaukee using interchangeable 5 amp batteries. Apparently, there are now batteries with 10 amps which is truly mind boggling. He recharges them using his solar panels, essentially making his house build powered by the sun. Well, almost, he’s got a generator for some bigger tools.

The issue of rare metal extraction and battery disposal at the end of its life is still something that I need to see solved before I dive into it, and a careful calculation of its environmental cost vs. its benefit has to be done. But the power and durability of these tools are now up to a point where I’d be quite happy to switch completely.

A small shed of tools is all you need to self-build a straw bale house.

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