Sustainable construction

Out of a Pippi Longstocking movie

The finished house doesn't look like hemp on the outside. It's clad in wood, with a porch and a green roof, like something out of a Pippi Longstocking movie. The construction does not work without concrete, but the material is only in the base plate and the supporting frame. Similar to half-timbered construction, this "skeleton" is lined, in this case with bricks made of pressed hemp straw bonded with lime mortar. The construction costs are about 15% higher compared to solid construction with the same equipment.

Since the house was completed, more than a thousand interested people have visited it. IN June, more than 700 people came to an "open weekend" alone. The next project will be a hemp eco-settlement with small residential units, a community room, car sharing and lawn mower sharing. Slowly, the developer is enjoying it, too. "Sustainable construction has now become a matter of conviction for me," he says.

Industrial hemp is just one of many suitable renewable building materials – along with wood, straw, or mushrooms, for example. What's new is that hemp houses no longer look like a dropout hut, Tiny House, or eco-village on the outside. This house looks like an ordinary wooden house and does not stand out much in a normal housing development.

There are some advantages. You build brick by brick – the masons and solid builders wouldn't have to learn entirely new techniques – though there are alternative construction methods using hemp and clay or timber frame construction. At the end of the house's use, perhaps after 70 or 100 years, there is no waste. The hemp and lime portion of the house can, in principle, be composted.

The ideal crop

The climate balance is good. A house made of hemp blocks, like a wooden house, binds carbon dioxide (CO2) as long as the house stands, and the material does not burn or rot. Climate researchers have been favoring wood as an ecologically compatible, regionally available building material for some time. It could even help countries achieve their 1.5-degree climate target, international researchers calculated in a study published in Nature in 2020.

Yet wood is already very expensive today – partly because of the rapid increase in demand. And it is questionable whether the resources of forests and plantations can suffice for a timber construction offensive on a global scale. In contrast, new construction raw materials are growing faster and more in the field. Anything that contains a high proportion of cellulose is suitable for construction.

Sustainable construction has now become a matter of conviction for me

European farmers harvest more than six tons of hemp straw per hectare. Polish researchers discovered that one ton of dry matter "stores" one to just under three tons of CO2. Hemp shives could be used particularly well as a building material "as a loosely filled thermal insulation material in timber frame constructions," writes the group led by Piotr Kosinski of the University of Olsztyn. A resulting increase in demand for hemp could also spice up agriculture ecologically because it can be usefully integrated into crop rotations. Its deep roots loosen the soil and increase its ability to absorb water. And winter varieties are also available.

Other ecological advantages for cultivation include low fertilizer and pesticide consumption.

History of hemp

But wasn't hemp a drug? Commercial hemp – unlike cannabis – contains very little to no THC, the active ingredient that produces intoxication. In Europe, it has a much older tradition as a textile fiber for cloth and sail making. And the first industrial use of hemp, beyond the textile processing practiced for centuries, dates back to entrepreneurs in the U.S. state of Kentucky. Almost 100 years ago, the Kentucky-Illinois Hemp Company invested in industrial hemp processing – including, for example, into bioplastics.

Henry Ford had his automotive engineers experiment with bioplastics that contained hemp fiber as well as soy, grain, and wood cellulose. Ford presented the prototype of the "soy car" in 1941. It contained a proportion of hemp fiber, was lightweight and economical. However, houses made of hemp did not exist at that time, but houses made of straw certainly did.

Then, however, hemp fell out of use. In the post-war decades, under the influence of drug-addicted youth cultures, America and Germany banned all cultivation of "marijuana", including industrial hemp plants. And suddenly there was an end to soy cars and hemp textiles. Interest also waned because concrete, steel, and other materials, such as cotton from Asia, were cheap and plentiful.

In the meantime, there is a worldwide shortage of building materials. The fact that hemp is being rediscovered for building is also due to new findings in building materials research: mixed with lime, the material continues to harden for years after construction. This is why the artificial word "hempcrete" was formed in English – for hemp ("hemp") and concrete ("concrete").

The hempcrete blocks are based on cellulose-containing residual materials from the fiber processing of industrial hemp, the shives. They are mixed with lime as a binder. Hemp lime is available as building blocks, but also as insulation boards or as insulating plaster. The building contractor Christian Eiskamp chose the brick form so as not to shock his employees. Most of them are bricklayers.

Rising demand but low cultivation

Building from algae

Another option is to mix concrete with hemp. This gives "higher flexural strength, lower density, lower thermal conductivity, higher flexibility, better durability and resistance to sulfate attack," as Indian scientists summarize in a recent review paper. Most interesting in this is the significantly improved insulating value. Just one percent hemp content in concrete reduces thermal conductivity by one-third compared with pure concrete, the study found.

This does not mean that there are no "sustainable" alternatives that have at least as good properties. "Classic" facade insulation made from recycled materials, for example: a 2016 French study found an insulating material made from recycled PET bottles superior to hemp in terms of the building's energy requirements, electricity consumption and living comfort. Possibly even more interesting seems to be the numerous projects around the world to turn concrete from a climate problem into a store of renewable carbon.

The University of Colorado is relying on microalgae, so-called coccosphaerales, which produce calcium carbonate from sunlight and CO2. "We envision a future where building with this cement will heal the planet," says Wil Srubar, an engineering professor at Boulder. Among other things, it is unclear how much material will be available and at what price.

However, hemp houses already exist. In the Netherlands, a company entered the market in 2018 with the first prefabricated house made of hemp. In France, the municipality of Croissy-Beaubourg near Paris had what is said to be the first municipal building in France to be built from hemp stones, a sports hall, in 2021. The Belgian hemp brick manufacturer Isohemp recently opened a second production line due to high demand. The company wants to produce an additional five million hemp bricks a year there.

There is a worldwide shortage of building materials

But how much hemp is available as a raw material in Europe? Farmers have so far reacted sluggishly to the rising demand, and cultivation is still low. According to Eurostat, the area under cultivation of industrial cannabis rose from around 20,000 hectares in 2015 to 36,000 hectares in 2020. The amount of raw material cannot simply be multiplied in this way, especially since grain cultivation is once again taking priority in agriculture.

The German-speaking hemp farming community is small and alternative. Similar to the early years of organic farming, there are recognizable cultural hopes associated with this movement. They aim for a more cooperative, artisanal, and regionally networked construction industry.

After all, it is still unclear how the ecological balance sheet would develop if the building material one day came from Morocco instead of Germany, Poland, or Belgium. There, in the Rif mountains, which are ideally suited to the climate, farmers would certainly like to grow more building material hemp than drug hemp – at least if the prices were right.