Not Just a Pipe Dream: Hemp as a Building Material by

After the U.S. government made it legal to grow industrial hemp late last year, a startup has announced that they’re building a factory to turn hemp fiber into “wood.” The factory, located in western Kentucky, will start turning hemp fibers into flooring and blocks by the end of the year.

HempWood is an impressive display of hemp’s versatility, but it’s not the first time the much-misunderstood plant has been used in building. Despite the legal challenges, people have been building with hemp for thousands of years.

What Is Hemp?

Hemp is the common name of a particular strain of Cannabis sativa grown specifically for industrial use. Although its cousin plants are harvested to produce marijuana, hemp plants have a far lower concentration of THC, which is the main psychoactive component of marijuana (approximately 0.3 percent). “You could smoke a telephone poles worth of our stuff and still not get high,” Ken Anderson, whose company uses hemp in building, told the New York Times .

What it lacks in recreational value, it makes up for in usefulness. Hemp makes two different kinds of fiber: the fine bast, which is used for paper and fabric, and the woody hurds of the inner stem. The hurds (also known as shives) are strong enough that they can be used in building.

Hemp has other properties that make it a good building material. A hemp crop can be ready to harvest in 4-6 months, and its roots grow densely enough that weeds aren’t a problem. For ecologically-minded builders, hemp also sequesters carbon from the air during its growth period: a hectare of hemp absorbs 4 times more CO2 during its lifetime than the same tree forest area .

Hemp’s useful building properties have been recognized for thousands of years. A 6th century bridge abutment in France was recently found to have been made with hemp-filled mortar. More recently, French restorationists developed a hemp-lime mixture to replace wattle and daub when restoring medieval timber-framed buildings. The mortar was an excellent replacement for Portland cement, which didn’t “breathe” like the wattle and daub had, thus causing the timber to accumulate moisture.

But hemp has remained a niche material because of its notorious family. In the 1930’s, the American government classed hemp under the Controlled Substances Act, meaning that it was regulated as a drug and could not be legally grown in the country. The classification meant that any hemp used in building had to be shipped in, and was subject to heavy scrutiny from permit issuers.

Despite the legal and financial barriers, pioneers still forged ahead working with it: there are approximately 50 homes in the U.S. containing hemp as a building material.

In the modern age, the most popular hemp-based building material has been hempcrete, a refinement on the hemp-lime mortar used in French restoration work. Hempcrete is an infill material made from hemp hurds mixed with a lime-based binder and water. This is possible because hemp hurds have an unusually high silica content for organic material, allowing it to bind to the lime with the same kind of calcium silicate hydrate (C-S-H) bonds found in hydrated Portland cement.

One thing that differentiates hempcrete from actual concrete is its density. The binder portion of hempcrete isn’t meant to fill all the voids between the hemp, just to stick them together where the fibers touch. Hempcrete is only about fifteen percent as dense as concrete, and cured hempcrete blocks will actually float in water. Because of this, hempcrete must be used with a frame that supports vertical load, like wooden stud framing.

On the other hand, the significant portion of trapped air in hempcrete makes it good insulation. It’s relatively breathable, but a good insulator for both heat and noise. The material is also resistant to mould , and fire-resistant. “We heat [our material] up to over 1,500 degrees Fahrenheit and it barely has an impact,” says builder Mark Faber, of Canadian hempcrete company Just BioFiber. “Very unlikely for this house to catch fire.”

In the last five years, more companies have stepped up to the plate on building with hempcrete, especially outside of the U.S. One particularly innovative company is Just BioFiber, which started making prefab building blocks from hempcrete in 2014. Their most famous project is the Harmless Home, a B.C.-based home custom-designed to be ecofriendly. The ecologically-friendly nature of hempcrete drew the homeowners toward picking it, as did its sturdiness. “Limestone houses last forever,” says homeowner Arno Keinonen.

The Harmless Home is a luxury home and, currently, building with hemp is significantly more expensive than more traditional materials. But a recent American legal development may open the door for less expensive hemp houses, and different ways of making them.

The most recent round of innovations in hemp architecture was spurred not by science but by a legal change. On December 20th, the U.S government designated hemp as an “agricultural commodity,” removing it from the scope of the Controlled Substances Act. Specifically, hemp with a THC concentration of not more than 0.3% is now legal to be grown in the U.S., and can be insured and treated like any other crop.

The news attracted the attention of Greg Wilson, owner of startup Fibonacci. Wilson started his career developing bamboo in China and co-owns SmartOak, a Tasmania-based company that makes “engineered wood” from wood pieces too small to make planks from. SmartOak takes wood strands, particles, fibers and flakes from these smaller logs, and bonds them together with adhesive.

Wilson looked for a plant fiber that could be used to replicate the SmartOak process without wood and started experimenting with hemp in the U.S. after a 2014 farm bill allowed for research with it. He developed a product called HempWood, which he says is a more sustainable alternative for hardwoods like oak. According to him, his processing algorithm can “reverse engineer” the hardness, density and stability of specific kinds of wood, and mimic them with plant fibers and “protein-based bonding agents.”

“The final attributes of a plant are based off of mainly the density, so when you have a more dense wood it is a harder more stable product,” Wilson told Maryland-based radio station WKMS . “So, because the density of fast-growing plants, which are more eco-friendly and is typically lower, we have to increase that density. In order to do that, you need to fill the voids in the cell structure of the plant, so we use adhesives.”

The result is an engineered wood that Wilson says is 20 percent denser than oak. That’s good news for both ecological and economic reasons, since a hemp crop takes 4-6 months to mature to an oak tree’s 60 years.

Wilson has announced that the company is leasing an 11,000-square-foot facility in Kentucky, and that they’re already cultivating more than 40,000 acres of hemp. The company’s website says that they’ll be ready to produce items like flooring, blocks, boards, and finished items like cutting boards and skateboards by summer 2019. “We look forward to being a productive member of Kentucky’s agricultural and manufacturing communities, and the enormous opportunities of HempWood as a renewable alternative to Oak,” Wilson says.

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