When you think about the materials that make up your home, oil probably isn’t at the top of your list. There’s a good chance it should be, however.
In a recent book about natural building materials, the leaders of New York design firm LTL Architects discussed the growing problem of fossil-fuel-derived products in residential construction. “The over 1 million homes which are built each year in North America are increasingly assembled and glued together from petrochemical-based materials throughout, from vinyl floors and siding to plastic lumber, acrylic paint, and asphalt shingles,” they wrote.
Petrochemical dwellings are only part of the problem. Globally, the building sector is a major source of planet-warming gases, and materials play a significant role. According to the World Green Building Council, construction materials and processes are responsible for 11% of all energy-related emissions. Three materials — concrete, steel, and aluminum — account for 23% of all emissions.
The advantages of building with hemp
For a small but growing community of building professionals, part of the solution lies in an unlikely source: hemp.
Kaja Kühl, a designer whose firm youarethecity used hemp in the construction of two micro homes in New York’s Hudson Valley, is a fan.
“As an architect, I see hemp, together with other biogenic [i.e., natural] materials, as building blocks to transition our design and building practices towards a regenerative post-carbon built environment,” she wrote in an email.
Ryan Doherty, vice president of the U.S. Hemp Building Association trade group, said hemp can be used in a wide variety of building products, from outdoor cladding to acoustic panels. “It’s just like a tree — it can be used like wood for so many other applications,” he said.
What is hempcrete?
Today, the most common hemp-based construction material is hempcrete, an insulation product that can replace products like fiberglass, rockwool, and foam. Hempcrete is created by combining the hemp plant’s stalk, which resembles balsa wood, with water and lime, a common mineral. This wet mix is then sprayed or packed onto a structural frame on the building site — hempcrete isn’t a structural material — or turned into blocks or panels that can be installed after they dry. Once the hempcrete is in place, builders often coat it with plaster to create a durable wall finish.
A long list of features makes hempcrete attractive for use in buildings. Nontoxic and biodegradable, it’s resistant to fire, mold, and pests. It can also help maintain comfortable interior temperatures, lowering the need for energy-intensive mechanical heating and air conditioning.
“It has thermal regulating properties which allow it to absorb moisture and heat and then distribute that heat or moisture over time,” said Jennifer Martin, a design/build professional who has worked extensively with hempcrete. “So what you’re left with is a very even indoor air temperature. You have even indoor humidity levels regardless of what’s happening outside.”
As concerns over the climate impact of building materials have grown, another benefit of hempcrete has risen to the top of the agenda: carbon storage. “Having worked in the green building community for 15 years, we weren’t necessarily talking about carbon storing with straw and hemp when I first started, but we absolutely are talking about it constantly now,” said Anthony Dente of Berkeley, California, engineering firm Verdant Structural Engineers, which promotes the use of natural materials.
Like other plants, hemp absorbs carbon dioxide during photosynthesis. But because it grows particularly quickly, rising to as much as 15 feet tall within 120 days, hemp can take in even more carbon than trees on an annual basis. Turning the stalk into building materials can capture this carbon and store it for as long as the structure remains intact.
You can’t smoke the house
Building with hemp isn’t a new idea.
“This is a historic material that’s been lost and refound,” said Martin. “There’s examples of hempcrete that are hundreds and hundreds of years old.” In the past few decades, France, Germany, and Canada have made strides in hempcrete adoption.
But in the United States, the field has been stunted by the long prohibition on growing hemp stemming from the Marihuana Tax Act of 1937. This ban was reversed in the 2018 Farm Bill, which also removed the plant’s seeds from the U.S. Drug Enforcement Administration’s list of controlled substances.
Confusion around the relationship between marijuana and hempcrete still affects the industry, Doherty said. Although hemp building materials have no psychoactive properties, “everyone thinks that we can smoke the house,” he said.
In fact, marijuana and hemp building products come from different families and parts of the hemp plant. By law, the hemp variety grown for building materials, which is referred to as industrial hemp, must contain less than 0.3% of THC, the chemical that produces marijuana’s high, when dried. Moreover, marijuana comes from the bud of the female flower, while hempcrete is made from the stalk of the plant.
How common is hemp construction?
Fewer than 100 hemp homes have been built in the United States, according to the U.S. Hemp Building Association, although interest in the material is growing. But scaling up the production of hemp buildings will require overcoming several hurdles.
One is a limited domestic supply chain. In 2022, only 18,251 acres of industrial hemp were harvested across the country, according to the Department of Agriculture.
“We’re at a very small scale — just a tiny crop in the United States,” Doherty said. As a result, some hempcrete buildings have been built with material imported from Canada and Europe.
But this is changing.
“With the legalization of [hemp] farming now behind us by a number of years, there is enough infrastructure and byproduct available to support offshoot industries like hempcrete building,” Dente wrote in an email. In recent years, companies like Kansas’s South Bend Industrial Hemp Processing have begun supplying builders around the country.
Doherty said scaling this up could benefit the nation’s rural communities as well as the climate. “You’ve got to process [hemp] where it’s grown and manufacture it where it’s processed to make [hempcrete] as cost-conscious as possible, so yeah, it’s definitely a rural job driver,” he said.
How much does hempcrete cost?
Domestic hempcrete production can also help more people afford hempcrete buildings. Not long ago, hemp houses could cost up to 30% more to build than conventional structures, Martin said. But as new U.S. hemp processors come online, reliance on expensive imported materials has fallen, lowering costs. And although the average hempcrete home is still more expensive than a business-as-usual equivalent, a hempcrete building program at the Lower Sioux Community in Minnesota has shown that it’s possible to achieve notable cost savings with the material, she said.
Another barrier to scaling hempcrete construction is the scarcity of builders and designers who have experience with it. Since the material is so novel, projects require much greater communication across the team, said Kim Croes, the founder of Detroit natural building company Fiber Fort. For example, workers can’t simply place two-by-fours at 16-inch intervals for the timber framing as they would for a typical project, since sprayed hempcrete requires a different configuration.
“It’s not just, we go in there, we do our job, and we leave, because our job affects everyone else’s job,” she said. “So I have to then explain what changes need to happen for the other trades.”
Is hempcrete accepted in building codes?
The U.S. hempcrete industry hit an important milestone in 2022 when it was accepted into the International Residential Code, which spells out rules for the construction of single- and two-family homes in 49 states. Inclusion in the International Residential Code makes it easier for builders to use the material without having to obtain special permission from their local building department. “It smooths out the permitting process and makes it easier to bring this building method into the mainstream,” Doherty said.
Dente, who worked on the code submission, cautioned that hempcrete is included only in the International Residential Code appendix, not the main document — a significant distinction, as appendices aren’t automatically adopted in all states that use the code. In addition, the code can’t be directly applied to commercial projects or other structures falling outside the jurisdiction of the International Residential Code, as these are governed by a separate document, the International Building Code. He and other hemp construction advocates are continuing to advocate for additional code changes to cover a broader range of buildings and geographies.
Despite these limitations, the official recognition for hemp is a major win, Dente said. Even in jurisdictions that don’t follow the International Residential Code appendix, and for building types not covered by the code, the appendix allows builders to point to an authoritative source declaring that hempcrete meets the intent of the building code — which can expedite the permitting process.
“There is still a lot of space to grow for the codification of hempcrete, but this is a huge first step for the material and making it accessible for people,” he said.