One of the oldest and most sustainable building materials of all time is staging a comeback: hemp. Now that several states have legalized the use of marijuana for some recreational and medicinal purposes, a big untapped market is emerging for cannabis to be used as a building tool. Across America a grassroots effort is underway among builders, architects, material suppliers and farmers to renew this fledgling market. Mixing hemp's woody core with lime and water produces a natural, light concrete that retains thermal mass and is highly insulating. No pests, no mold, good acoustics, low humidity, no pesticide.It grows from seed to harvest in about four months. A catalyst that may spark growth in hemp home building is Senate Majority Leader Mitch McConnell's, R-Ky., Industrial Hemp Farming Act of 2018, introduced last week. If passed, the bill — co-introduced by Sen. Rand Paul, R-Ky., Sen. Ron Wyden, D-Ore., and Sen. Jeff Merkley, D-Ore. — will remove industrial hemp from Schedule I of the Controlled Substances Act and allow it to be regulated as an agricultural crop. The bill will also authorize and encourage access to federal research funding for hemp and remove restrictions on banking, water rights and other regulatory barriers the hemp industry currently faces. A companion bill has been introduced in the House. All this has Joy Beckerman, president of the Hemp Industries Association, a D.C.-based nonprofit trade association, very excited. "We've increased hemp agriculture in the U.S. to more than 25,000 acres," she said, "and we hope to triple it this year." (Last year Colorado was the top hemp grower, at nearly 9,700 acres, followed by Kentucky, at around 3,100 acres.)

Main entrance of the Maui hemp house. Source: American Lime Technology (Builder: George Rixey, Rixey Co.; Photographer: Travis Rowan, Living Maui Media)

For years this lanky, woody weed has been poorly misunderstood. Like marijuana, its evil twin, it is of the Cannabis sativa species. But that's where the similarity ends. While marijuana is largely used for medicinal and recreational purposes, hemp is cultivated for its fiber, hurd and seeds and contains just 0.3 percent of tetrahydrocannabinol, or THC, the compound that gets you high. Today at least 25,000 products — including apparel, foods, plastics, skin products and dietary supplements — are made of hemp, and now, with the rising demand of nature-friendly construction, it's gaining attention worldwide for its use as an alternative to the ubiquitous fiberglass insulation in homebuilding. Hemp-based structural blocks and prefabricated panels for exterior walls are also being introduced into the market. The reason: It's nontoxic, fireproof, carbon-capturing, mold- and pest-resistant. Hempcrete, also known as hemplime, is a biocomposite made from the inner woody core of the hemp plant that is mixed with a lime-based binder and water. The result is a lightweight insulating material weighing only about an eighth of the weight of concrete. In the United States there are currently only about 50 homes containing hemp, in such states as North Carolina, Virginia, Texas and Hawaii, but this natural wonder has been used in construction around the world for centuries. Today it can be found in hundreds of homes and commercial buildings in Canada and Europe, including an eco-house built by Prince Charles.

Front of the Maui hemp house facing the Pacific Ocean. Source: American Lime Technology (Builder: George Rixey, Rixey Co.; Photographer: Travis Rowan, Living Maui Media)

Unlike the Three Little Pigs' domiciles, made mainly of straw, sticks or bricks, these aren't literally hemp houses. They're conventionally designed and built by conventional, if adventurous builders. The homeowner, however, opts to use hemp-based materials for insulation and/or exterior or interior walls as alternatives to customary fiberglass or foam, concrete and wood. Foundations, framing, electrical, plumbing and HVAC can be standard or incorporate "green" materials and systems. A hemp-savvy architect, engineer or consultant from a hemp products maker is often involved, because the materials have distinct specifications and construction requirements. Hemp is adaptable to any climate, and though it can cost slightly more than typical houses — especially if it's imported — proponents claim the long-term ROI is worth the bump. That's because homeowners enjoy long-term energy savings and sometimes lower insurance premiums. More from the Business of Design: Amazing products made of trash

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The Hemp Business Journal and Vote Hemp, a legislative advocacy group, estimate that the total retail value of all hemp products sold in the United States in 2016 was $688 million, predicting revenues will reach $1.8 billion by 2020. Industrial product sales, including hemp-based building materials, reached $125.5 million in 2016. It's safe to say that hemp-building products represent a niche market within the wider green building materials industry, which posted $86.6 billion in revenues last year, according to Freedonia Group.

A contemporary showpiece

Hemp Technologies Global in San Diego was involved in what's claimed to be the first modern hemp home in the United States, built in 2010 for Russ Martin, the then-mayor of Asheville, North Carolina. Building the 3,400-sq.-ft. contemporary-style showpiece cost $133 per sq. ft., or $452,200, which is on par with today's conventional rates. Martin reportedly sold the house last year for $685,000.

The first modern hemp house in the U.S., built in Asheville, NC, for the then mayor in 2010. Source: Hemp Technologies

"We supplied and mixed the hempcrete on site that was used for insulation," said company founder Greg Flavall, explaining part of the building process. That process begins well before any ground is broken. Because many local permitting and building department officials aren't familiar with hemp products, suppliers like Flavall spend time simply educating them about the materials and providing research data on their viability in terms of strength, thermal properties and other specifications. The same goes for engineers, architects and builders involved in hemp projects, said Luly Abraira, a principal at Asheville-based Hempsteads, formerly Alembic Studio, which designed and consulted on the so-called Nauhaus, a craftsman bungalow in town, as well as a classic two-story house in Virginia Beach. "From the outside it looks very normal," said owner Mike Meehan, who described his seaside home as a clean, green, energy-saving machine.

Beachfront house on Maui under construction, with first-floor walls made of hempcrete panels. Source: American Lime Technology

"The learning curve can be a barrier," Abraira said, "so there's a lot of handholding." There still aren't international standards for building with hemp, or codes regulating how it should be used structurally or safely. ASTM International, a technical standards organization, formed a committee to address this in 2017.

Hemp as a building block

Hempcrete is widely used as natural insulation between exterior structural walls built from wood, steel or concrete. It's airtight yet breathable and flexible, but not load-bearing. "When the cast mix cures, it can be finished in the inside with lime or clay plaster and masonry or wood on the outside," explained Mario Machnicki, president of American Lime Technology in Chicago, whose hempcrete products have been used in roughly 30 projects in the U.S. and Canada. Machnicki and other suppliers said that hempcrete can cost slightly more than conventional insulation, especially when the hemp hurd has to be imported. "When we started the business 12 years ago, all our hemp was imported from the U.K. and was expensive to ship," he said. As U.S.-grown hemp becomes more available, pricing for hempcrete will be even more price-competitive, he added. Higher costs, though, won't necessarily hinder the hemp house market. Already, a segment of the homeowner population has been willing to invest extra for solar power, geothermal heating and air-conditioning and other sustainable building materials, often with the expectation that they'll save money in the long run on energy costs. There's also the intangible value of being an innovator and early-adopter, not to mention the "cool" factor. Just BioFiber, founded in Calgary, Alberta, in 2014, has patented prefabricated, structural building blocks made from hemp. The interlocking blocks resemble Legos, said CEO Terry Radford, a former IT executive turned hemp entrepreneur. "Our first project is what the owner calls Harmless Home, built on Vancouver Island," he reported. Scheduled to be completed this summer, the sustainable, off-the-grid custom-designed home features solar electricity and hot water and a hydroponic greenhouse for growing organic fruits and vegetables.

Lego-like hemp blocks from Just BioFiber fit together to form the Harmless Home’s exterior. Source: Arno Keinonen

No doubt, the hemp homebuilding sector is still in its nascent stage. No major homebuilding companies specialize in using hemp, so it's more of a one-off situation for local designers and builders. To that point, a media representative for the National Association of Home Builders, told CNBC that "unfortunately, we don't have any information on the use of hemp in residential construction."

The holdup on this side of the Atlantic

The holdup on this side of the Atlantic is a legal limbo, combined with a lack of availability of raw hemp and general awareness about hempcrete's qualities among consumers, homebuilders and municipalities that issue building permits. "There's still a lot of 'Reefer Madness' out there," lamented Erica McBride, executive director of the National Hemp Association, a grassroots organization based in Washington, D.C., that advocates for the use of hemp in foods, medicines, clothing, paper, auto interiors and building materials. McBride's maddening reference was to the 1936 exploitation film that demonized pot as "the deadly scourge that drags our children into the quagmires of degradation." A year later Congress passed the Marihuana Tax Act of 1937, which essentially outlawed growing either plant. In 1972 the feds classified all cannabis as a Schedule 1 controlled substance, making it as illegal as heroin and LSD.

The Harmless Home on Vancouver Island, built with hemp blocks, is scheduled for completion this summer. Source: Arno Keinonen