Building with Hemp

Once buried by politics, hemp is reemerging as a hero of sustainable building.

Humans build lots of stuff. We need sustainable materials more than ever. We need building materials that do less damage to the natural environment during their production and eventual disposal. Several new building materials seek to do this, and one that checks a lot of the boxes for both sustainability and favorable material properties is hemp.

Let's just clarify something. Hemp is a non-psychoactive industrial crop from the cannabis sativa species, grown for its fibers and seeds. Used for building materials and human food, industrial hemp has tall skinny stalks and has a negligible amount of THC—less than 0.3% in the U.S. under the 2018 Farm Bill. Marijuana, on the other hand, is a low bushy variety grown for compounds in its flowers, which contain 5-30% THC plus other compounds for medicinal or recreational use, and is not used for industrial construction.

History of Hemp

Hemp has been used as a versatile material for thousands of years. In 8,000 BC, the Mesopotamians used it to make cord for pottery. Ancient civilizations in China and Kazakhstan used it for textiles and the earliest known paper around 200 BC. In medieval Europe, hemp was essential for making rope, sails, and clothing—famously used on Columbus's ships and in Gutenberg's Bible.

Hemp has deep roots in American history, tracing back to the 1600s when colonial governments mandated its cultivation for rope, sails, and textiles. Early leaders like George Washington and Thomas Jefferson grew it on their farms. But in the early 1900s, steam power began to replace hemp for sails, and the invention of the cotton gin made processing cotton more economical than hemp.

A turning point came in 1916 when G.W. Schlichten invented a machine that could have revolutionized hemp processing. Similar to the cotton gin, it reduced the labor required to process hemp into products by a factor of 100. It was around this time that racist fears tied to cannabis use led to bans and public stigma, making investors reluctant to fund the production of Schlichten's invention.

The 1930s “Reefer Madness” campaign further associated hemp with marijuana, culminating in the 1937 Marihuana Tax Act, which taxed all cannabis products into near extinction. These campaigns and attacks on hemp were perpetuated by corporate incumbents like media mogul William Randolph Hearst because his paper companies were being replaced by hemp. Similarly, chemical giant DuPont's investment in nylon was threatened by hemp products.

Although briefly revived during WWII for military use, industrial hemp effectively disappeared in the U.S. by 1958. The U.S. didn't start issuing licenses to grow hemp until 2007, and it wasn't until 2018, with the passage of the Agriculture Improvement Act, that the U.S. government defined industrial hemp as a federally legal plant. Hemp went from a Schedule 1 drug to a commodity crop. It's now valued for sustainable materials like hempcrete, bioplastics, and eco-friendly fabrics.

Modern Uses of Hemp

The hemp plant is a fast-growing annual crop that thrives in temperate climates with well-distributed rainfall. Hemp is usually planted in late spring when soil temperatures reach at least 50°F. Depending on the variety and purpose, it matures in about 70 to 120 days and can grow between 6 and 15 feet tall.

Hemp can be grown for different uses, including fiber, seed, or CBD. The stalk contains two main components: the outer bast fiber and the inner woody core known as the hurd or shiv. Bast fibers are used in textiles, rope, and insulation, while the hurds are commonly processed into hempcrete, particleboard, and animal bedding. The seeds are harvested for food products like hemp hearts and oil. While leaves and flowers are primarily used for CBD extraction, they are not typically involved in industrial material applications.

Harvesting depends on the crop’s end use. Hemp for fiber is usually cut when plants are tall but before seeds form, then left in the field to ret (a natural process that helps separate the fibers). Seed crops are harvested later using combines. After harvesting, the stalks go through decortication, where machines separate the bast fibers from the woody hurds.

Hemp is, of course, renewable. It requires similar amounts of fertilizer as other crops. It's naturally resistant to most pests and diseases, so pesticide use is lower than in other crops. Hemp’s dense canopy also helps suppress weeds, reducing the need for herbicides. Because of its low input requirements, rapid growth rate, and ability to sequester carbon, hemp is considered a sustainable crop.

Biomass Production Rate and Crop Rotation

The rate at which a plant produces biomass is proportional to the amount of CO₂ it removes from the air. For every kilogram of plant biomass production, roughly 0.5 kg of carbon is stored, which is about 1.8 kg of CO₂ removed from the air.

Tree plantations of spruce and Douglas fir grown for timber produce an average of 300-500 tonnes (that's metric tonnes) of wood per hectare at harvest after 40 years of growth, which equates to 7.5-12.5 t/ha/yr on average.

Industrial hemp can produce about 6-15 tonnes of biomass per hectare per harvest. There's usually only one harvest of hemp possible per year, but in some locations, "double cropping" can be achieved. For instance, in regions where winter wheat is harvested in early summer, farmers can plant hemp immediately afterward and harvest it before winter sets in. This rotation maximizes land use efficiency and can enhance soil health. A hectare of hemp can remove anywhere from 10-25 tonnes of CO₂ from the air per harvest.

That puts hemp's biomass production rate a little above that of spruce and Douglas fir plantations, and similar to switchgrass and miscanthus—all of which are used to make products for construction. Hemp has a higher cellulose content than wood, which is generally the most useful component of the biomass for making materials.

The fact that hemp grows so fast means that hemp farmers can recoup their investment quickly. Hemp can be added to the crop rotation of current agricultural fields, where it has been shown to reduce soil-borne diseases and increase the yields of crops like watermelon, potato, and beans. One disadvantage is that it requires relatively good agricultural land that can be used for other useful purposes, whereas trees for timber can be grown on hilly terrain unsuitable for agriculture.

Phytoremediation

Hemp is emerging as a tool in phytoremediation—the use of plants to clean up contaminated soil. It has been shown to absorb and neutralize a wide range of pollutants, including heavy metals, pesticides, solvents, explosives, crude oil, and polyaromatic hydrocarbons. Toxins can accumulate in the plant’s roots, leaves, and stalks, which is why only non-edible parts like the stalks are harvested for post-remediation use. These stalks can still be safely processed into biofuel, paper, textiles, or building materials.

Hemp grown on polluted land can also be distilled into ethanol, turning environmental cleanup into a source of renewable energy. There is even discussion of recovering heavy metals from the hemp plants for industrial reuse.

Hemp Building Materials

Over 30% of global energy use and CO₂ emissions are attributed to the construction sector, so sustainable building materials have a large impact. Hemp can be made into rope, paper, clothing, hempcrete, insulation, particleboard, acoustic panels, furniture, flooring, bioplastics, paint additives, and reinforced polymer composites.

Hempcrete

Hempcrete is a sustainable building material made by mixing ground hemp hurds or shiv with water and binders. Modern hempcrete typically uses hydrated lime, sometimes combined with hydraulic or pozzolanic binders, such as metakaolin, to speed up the setting time. Lime is produced by heating limestone (CaCO₃) to create quicklime (CaO) through calcination. When mixed with water, quicklime forms hydrated lime (Ca(OH)₂).

Hydrated lime absorbs CO₂ from the air as it cures:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Originating in France in the 1990s as a bio-based alternative for masonry, hempcrete produces a solid but lightweight and durable material, with densities ranging from 200 to 800 kg/m³. Its low thermal conductivity (0.05–0.14 W/m·K) gives it great insulating properties. Its R-value is between 1.0 and 2.9 per inch, depending on density, so a 12-inch-thick wall can easily achieve R-24 or higher.

The lime is naturally alkaline and deters pests and inhibits mold or microbial growth. Hempcrete also has high moisture permeability and fire resistance, acting as a fire separation medium for up to two hours. A 260 mm thick hemp-lime wall can sequester up to 35 kg of CO₂ per square meter over 100 years and require 30% less energy than concrete, while an equivalent concrete wall will release 52 kg of CO₂.

Hempcrete has been around for over 30 years but hasn't caught on widely due to regulatory hurdles, higher costs, lack of familiarity, and limited supply chains. Since hempcrete is non-load-bearing, it needs a steel or wood frame. It also takes longer to install than traditional construction, and many builders don't want to spend the time. But with growing interest in low-carbon construction, inclusion in building codes, and increasing investment in hemp infrastructure, hempcrete is being used more.

There are many companies worldwide that offer hempcrete for sale—it's really just hemp hurds and lime—and many that offer installation. Some companies, such as Hemp Block USA make prefabricated hempcrete blocks that are quicker to install and fit together like Legos.

Insulation

To overcome the higher cost, slower construction time, and unfamiliarity with hempcrete construction, companies like Hempitecture are creating insulation materials from hemp that integrate easily into traditional construction methods and directly replace other toxic, unhealthy, and non-biodegradable insulations, such as fiberglass and foams. They make insulation in the form of hemp batts that replace fiberglass batts in between wall studs and joists, hemp panels that replace exterior foam boards for continuous insulation, loose-fill hemp that can replace blown-in fiberglass in attics, and even hemp-based carpet underlayment.

The hemp batts are over R-3.7 per inch, which is better than fiberglass. Fiberglass has dominated the insulation market for years, but it's made with carcinogens such as formaldehyde, and is unsafe to touch and inhale the sharp, tiny glass fibers, which become airborne easily and lodge in the lungs. These hemp-based insulations are VOC-free, non-toxic, and safe to handle without protection.

Flooring and Furniture

Hemp can be made into beautiful flooring and furniture. HempWood is leading the way here with their hemp-based flooring composed of hemp stalks and organic binders pressed together. With a Janka hardness rating of 2,200 lbf, it's 60% harder than oak and 20% harder than hickory.

HempWood can replace wood in many applications, such as furniture and cabinetry. HempWood products are zero-VOC and help create healthy indoor environments.

Conclusion

We have a strong need to increase the sustainability and decrease the toxicity of the world's building materials. With the number of buildings humans have built on Earth and the ones we continue to build, it's crucial to use materials that do not harm human health and are ecologically benign. No building or house stands forever. We need building materials and insulations that can be biodegraded harmlessly or recycled infinitely at the end of their lives. Most cannot. Foam insulations, fiberglass, and formaldehyde-based OSB and plywood are impossible to recycle or safely return to Earth.

Hemp's rapid growth, low input requirements, and ability to sequester carbon make it one of the most environmentally friendly crops available. Its versatility as a building material makes it a good replacement for traditional wall assemblies, insulations, flooring, furniture, and more. As investment in hemp infrastructure increases and more manufacturers enter the market, hemp materials are becoming more accessible and practical. If you are building a house or are installing new flooring, consider using hemp products from companies like Hempitecture and HempWood—even if for no other reason than that they're healthy and beautiful.

Questions for you:

• What are the most important factors you consider when choosing materials for construction or remodeling?

• Are there any other advantages or disadvantages to hemp-based building materials?

• What would prevent you from using hemp building materials?

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