It was used to seal Chinese sailing vessels in 200 BCE, and to reinforce bridges when France was still Gaul. Hemp is one of the fastest-growing plants on the planet, and its potential as a building material has been known for centuries.
A strain of the ubiquitous Cannabis sativa (yes, it is the same one that produces marijuana, but it cannot contain more than 0.3 percent of THC), hemp can grow up to 13 feet in a matter of months. It can be cultivated in 90 to 120 days, which is 100 times faster than oak trees. It is lighter and less expensive than wood, and according to recent studies, it can capture carbon twice as effectively as a forest of trees.
Building materials are responsible for 11% of global carbon emissions, and as the building industry continues to look for ways to lower its carbon footprint, scientists, architects and manufacturers alike are turning to natural materials. Together with other biomaterials like algae, mycelium, or even coffee husks, hemp is gaining popularity as one of the world’s most sustainable materials—if only we didn’t have to jump through regulatory hoops to grow it.
This summer marked the completion of France’s first public building to be made of hemp. Designed by French studio Lemoal Lemoal, the 4,000 square-foot facility, called the Pierre Chevet Sports Center, sits in the town of Croissy-Beaubourg near Paris. Its walls are infilled with hempcrete blocks, then clad with cement-fiber panels to protect the hemp blocks from the elements. The hemp panels were grown and fabricated within 310 miles of the site.
Hempcrete is made from mixing hemp with lime and water. It can weigh about an eighth of regular concrete, but the architects said they chose it for its acoustic and thermal properties as well its resistance to fire. They said the main challenge with hemp is to convince clients that it’s a viable alternative to concrete, especially when a hemp wall looks a little more rustic and not as refined as concrete. It doesn’t help that hempcrete is currently more expensive than concrete, but in the long term, they said it can help building owners save on energy bills thanks to its insulation properties.
Today, China is the world’s largest producer of hemp, followed by France, the largest producer in the European Union. The U.S. is still catching up, in large part because hemp production was banned in 1937, when the Marijuana Tax Act was passed. (Before the ban, Henry Ford famously used hemp fiber composites in his first Model T car.)
In 2018, the U.S. Farm Bill was amended to legalize agricultural hemp, and the industry has slowly been growing. In Ketchum, Idaho, a startup called Hempitecture creates HempWool, a type of insulation made of 95% hemp fibers as well as hempcrete blocks. In Durham, North Carolina, Plantd is prototyping a hemp-based Oriented Strand Board (a kind of material often used to sheath roofs, walls, and floors.) And in Kentucky, a company called HempWood is using hemp fibers to manufacture alternative wood.
Gregory Wilson founded HempWood the same year hemp was legalized in the U.S. In two factories in Kentucky, he started with hemp-based lumber, but the business has since grown to HempWood flooring and furniture. In the coming years, Wilson plans to expand into outdoor decking, waterproof flooring and structural lumber.
The company has projects all over the country: from the dorm of a medical school in Louisville to corporate offices in New York City. HempWood flooring, the company’s most popular product, was even featured in BMW’s concept showroom in Los Angeles last month.
HempWood looks a lot like wood, but it has been rated 20% stronger than hickory, which is the hardest commercially available hardwood in the country. The manufacturing process is radically different, too. Once hemp is harvested, the company uses a soy-based adhesive to bind the fiber stalks, compress them, and bake the material to form HempWood blocks. (It takes about 150 days from the moment the seeds are planted to the moment the product is complete.)
Darshil Sha is senior researcher within the Centre for Natural Material Innovation at the University of Cambridge. He says hemp was traditionally used to make ropes, textiles, and paper, but its role as a building material is increasing. “The advantage of hemp is that it’s more likely to be sustainably managed than timber, depending on where timber is sourced from,” says Shah.
“Lots of innovations are going on now, looking at what forms hempcrete can take,” he says. You can make bricks out of hempcrete, you can 3D print with hempcrete, and you can even spray a mixture of water, lime and hemp inside walls in lieu of foam insulation.
Last year, Shah collaborated with the Irish-British filmmaker Steve Barron (who directed the music video for Michael Jackson’s “Billie Jean”). Together with London-based Practice Architecture, Barron converted his 53-acre Margent Farm in Cambridgeshire to hemp production and used the crops to build his own house. The walls are made of hempcrete, and the building is clad in a custom-made corrugated material made of hemp fibers and bio-resin. Shah says that envelope helped lower the building’s carbon emissions by 18-20%.
“There’s been a rethinking with hemp, mostly in the past five years, but it’s been going on under the current since the ’90s,” Shah says. For now, hemp can’t be used structurally or on its own. “Timber works as a good partner with hemp,” says Shah, but hemp-based creative solutions abound. Over the past few months, Shah has been researching ways to build wind blades out of hemp (currently, wind turbine blades are sent to a landfill). He’s also working on creating fully compostable hemp-bioplastics to be used in packaging.
“One of the things hemp sector and plant-based materials lack is the cohesiveness because it’s such a discrete industry,” says Shah. For hemp to really take off, he says regulations need to change. In the U.S., you need a special permit (courtesy of U.S. Department of Agriculture) to grow so-called industrial hemp. And in the U.K., where Shah is based, that permit is issued by the British Home Office, and drug laws are hampering the U.K.’s hemp industry. “When people say [hemp] is the plant of the future, that may be true in the future,” he says, “but we have a lot of catching up to do in terms of its production quantity.”