



This article first appeared in Cannabis Now Magazine issue four.

Long Dormant, the Ancient Technology of ‘Hempcrete’ is Making a Comeback

It sounds like some kind of futuristic technology: a super-strong, ultra-light building material which can be poured like concrete and cut like wood, endlessly recyclable and inexpensive to produce, made from renewable resources which reverse climate change as a side benefit to their production. It makes possible an entirely new generation of homes and other structures which provide superior insulation from the elements and even grow stronger after they’re built, because their lightweight material actually draws carbon dioxide out of the atmosphere and sequesters it inside, reinforcing itself from the air. Yet this futuristic-sounding technology could scarcely be older: the first people to build their homes from it were none other than the ancient Egyptians, and after them, the ancient Merovingians of southern France. Now known as “hempcrete,” this venerable technology is rapidly on the rise, especially in Europe where its durability and ecological benefits have garnered mass appeal.

Hempcrete is easy to produce. The ancient Egyptians, who grew cannabis for its fibers, quickly discovered that the plant’s ‘hurds’ (the hollow woody core in the interior of the trunk) could be equally useful. After stripping them of fibers, the Egyptians dried the hurds and ground them up into a lightweight powder which they mixed with lime, a cheap and plentiful mineral. The final ingredient also came from the hemp plant – from a sticky layer separating the bast fiber from the plant’s woody hurds. The Egyptians mixed this substance with water, pouring the goopy mixture into brick-shaped molds lined with straw and allowing it to set in the dry desert air. When the drying was complete, they were left with bricks which were stronger than rock-based concrete yet, incredibly, only one-sixth the weight.

In recent years, material scientists and engineers have begun to rediscover this valuable technology. The French in particular have taken to adopting the habits of their Merovingian antecedents. Years of French experience have confirmed that houses built from hempcrete are long-lasting and durable, keeping cool in the summer and warm in the winter without incurring high energy costs.

The Greenest Green

But for all its other benefits, perhaps the most revolutionary aspect of hempcrete is its superior environmental profile. Most homes in the U.S. today are built out of wood, with its attendant ecological costs — while no North American lumber company practices the kind of rampant clear-cutting which was the rule a century ago, the modern lumber industry still causes environmental harm by reducing forest habitats, contributing to erosion, and reducing the capacity of forests to combat global warming. But while cutting down trees causes demonstrable harm, the environmental benefits of growing industrial hemp have been well known for a hundred years. As far back as 1906, a U.S. government study found that, acre for acre, industrial hemp could sustainably produce over four times the usable biomass as forests or tree farms, a result of cannabis’s astonishing rate of growth (that study focused on the potential to replace wood-based paper pulp with hemp hurds, but the same benefits could be derived by replacing lumber with hempcrete). And U.S.D.A. experiments conducted at Arlington Farms in Virginia (the same plot of land now occupied, ironically, by the Pentagon) found that growing hemp eliminated the need for chemical herbicides or pesticides; required only small amounts of fertilizer; improved the state of the topsoil by making it loamier, airier, and deeper; encouraged growth of beneficial mycelium; and left plots of land free of weeds for the next crop in rotation.

While all of these benefits have garnered industrial hemp a well-deserved reputation for environmental friendliness, turning that hemp into hempcrete presents still more benefits, in the form of carbon sequestration. Trees, especially in the form of old-growth forests, have long performed this vital function, sequestering carbon dioxide from the atmosphere into their massive trunks and roots, or dropping the carbon to enrich the soil by turning their annual leaves into rich loamy mulch. Yet forests are threatened by clear-cutting worldwide.

Enter hempcrete. Studies of the hempcrete industry in France have shown that hempcrete production represents an excellent opportunity to sequester carbon on a mass scale, by sequestering carbon in three primary ways. The first way is a result of the cannabis plant’s rapid growth: in a single season, a cannabis plant can grow up to twenty feet (6.5 meters). If burned, most of the carbon from those hurds will be released back into the atmosphere; if left to rot, some of the carbon will gas off into the air and some will mulch into the soil. But tests have shown that if the hurds are converted into hempcrete, the long-lasting material will effectively lock the carbon within for the indefinite future. This it accomplishes not by sealing the carbon off from the outside air, but by doing rather the opposite.

This effect is, in fact, the second way in which hempcrete sequesters carbon: even after a hempcrete home is already built, its hempcrete walls continue to “breathe” the outside air. Hempcrete is porous, allowing carbon dioxide to waft deep inside its lattice structures, coming in contact with its “surface” deep inside the brick. Over time, some of this carbon breaks off from the two oxygen atoms and binds to the carbon-based lattice of the hempcrete, reinforcing the brick from within. The bricks then off-gas pure oxygen, mimicking the same process that the cannabis plant used to grow (albeit at a much slower rate). Thus, structures made with poured hempcrete continue to sequester carbon for up to twenty years after their construction, actually strengthening rather than degrading during that period.

But the third way hempcrete can sequester carbon may be the most profound: saving the forests of the world by eliminating the need to cut down trees. Switching to hempcrete for building materials, in combination with switching to hemp paper, could make forest destruction a thing of the past, allowing forests to progress to old-growth stage in keeping with the natural order of their ecosystems. Because old-growth forests sequester far more carbon than forests cleared every twenty to thirty years, eliminating the lumber industry may prove to be the most profound way an American hempcrete industry could reverse climate change.

Yet none of this is possible in the U.S., for the simple reason that federal law criminalizes all forms of the cannabis plant, regardless of whether it is grown for intoxication, medicine, fiber, seed, or hempcrete. Thus, any American committed to saving the forests should contact their Congressional representatives, demanding that, at the very least, they end the prohibition of industrial hemp, which produces less than 0.3% THC and couldn’t get a stoner high if he smoked an entire field of it. With a little tweak of federal law, sidewalks which reverse global warming may be just around the corner.