Building with earth: cob, rammed earth, hempcrete and the Brighton Earthship

Building with earth is one of the oldest ways of making a home, and one of the lowest-impact. It is also being reinvented, with hemp, lime and modern engineering. Here is what it can do, and what it cannot.

When people talk about a "new earth", I like to keep it literal for a moment. There is a very practical version of the idea: houses made from soil, straw, lime and plants, which barely need energy to make and can last for centuries. I find this more interesting than most futuristic architecture, because the materials are mostly under our feet.

Older and more common than you might think

Building with earth is not a niche. The commonly quoted figure is that about a third of the world's people live in homes built wholly or partly from earth, a figure repeated by the Getty Conservation Institute and in UNESCO material. UNESCO also runs a World Heritage Earthen Architecture Programme because a large share of the world's historic sites are made of earth. Treat the one-third figure as an estimate, since it is hard to count.

The main methods

• Cob: a mix of clay-rich subsoil, sand and straw, worked wet and built up by hand into thick, sculpted walls. It has a long tradition in south-west England and is easy to shape into curves.
• Rammed earth: damp subsoil compacted in layers inside temporary formwork, producing dense, smooth walls with a striped, layered look.
• Adobe or earth blocks: sun-dried bricks made from earth and straw.
• Straw bale: bales of straw stacked and plastered, valued for insulation.
• Hempcrete: the woody core of the hemp plant mixed with a lime binder, poured or sprayed around a timber frame. It insulates and is breathable.
• Lime plaster: the traditional finish for all of these, letting walls release moisture.

What earth is good at

The Centre for Alternative Technology (CAT) is a UK charity that has worked with these materials for decades. Its guide makes a few points I find useful:

• Earth is not an insulator, but it is good thermal mass: it soaks up heat and gives it back slowly, which smooths out swings in temperature. In UK climates it works best for internal walls, with proper insulation on the outside.
• It has very low embodied energy, because little energy is needed to dig it, move it and work it.
• It is breathable, especially with lime finishes, so moisture can move through walls instead of getting trapped.
• Hemp grows fast and stores carbon as it grows, which offsets some of the emissions from making the lime binder. CAT lists its thermal conductivity as roughly 0.06 to 0.11 W/mK, so it insulates more than earth alone.

What it is not good at

I would be doing you a disservice if I stopped at the good news.

• Water. Earth walls must be protected from rain and rising damp with good foundations, generous roof overhangs and breathable finishes. A poorly detailed earth wall can fail.
• Cement can undo the benefits. Adding cement to stabilise rammed earth raises its carbon impact, which is why some builders prefer un-stabilised earth where the soil and design allow.
• Regulations and insurance. In the UK, building regulations were written around bricks, blocks and timber, so earth and straw buildings often need extra engineering sign-off, and lenders and insurers may ask more questions. Codes vary from country to country, so ask locally.
• Skills and time. Cob and rammed earth take labour and craft. That can be a strength for a community project, and a problem for a fast, cheap build.
• Not every soil works. The right mix of clay, sand and silt matters, and it needs testing.

The Brighton Earthship

An Earthship is a style of house developed by the American architect Michael Reynolds: earth-sheltered, passively heated by the sun, using recycled materials such as tyres packed with earth for the walls, and designed to collect its own rainwater and treat its own wastewater with plants. Earthship Brighton was the first Earthship built in the UK, created by the Low Carbon Trust after Reynolds visited and spoke in Brighton in 2000. Sources differ on the date, saying it opened in 2005 and was completed in 2006. It is an off-grid building that is designed to heat, cool and power itself from the sun and to collect its water from the sky. You can read about it on the Low Carbon Trust's page and in the Wikipedia article.

I mention it mostly as proof that a strange-looking building can be made legal, insured and used in a British town. There has also been a public appeal to upgrade its energy and water systems, and I think that honesty is worth having: experimental buildings are living projects, not finished products.

Where earth building fits in a modern home

You do not have to build a whole cob house to use these ideas. Some realistic options:

1. Lime plaster and breathable finishes on an old solid-wall house, instead of cement or gypsum.
2. Hempcrete insulation in a timber-frame extension.
3. A rammed-earth or cob feature wall inside a house, for thermal mass.
4. An earth oven or a garden wall. A good first project, and low risk.
5. A workshop at a community garden or a local project, learning by doing.

Why I care about this

It fits the solarpunk idea very neatly: technology and nature working together, on a human scale, using materials close to home. It is also very good for a fast mind, because building with your hands gives your attention something real to work against, with clear progress and no screen. If you have ever felt stuck at a desk, a weekend of mixing and shaping earth is a different sort of focus altogether.

If you want to see what a courses and community route looks like, the ADHD MindFlow community is a good place to ask who has built or renovated with natural materials. And before a physical project, a short listening session can help you settle into it.

A personal view, with sources linked. Building regulations, structural safety and insurance vary by place, so get local professional advice before you build.