A wood substitute by HempWood
courtesy HempWood A wood substitute by HempWood

Concrete, steel, and wood comprise the triumvirate of structural building materials today. Most buildings contain all three in some capacity and employ one as a predominant framing medium. Architects and engineers default to these materials so regularly that the market seems impenetrable to alternatives. And yet, a fourth option may enter the picture—and become the next disruptive technology in building structure, after mass timber.

Hemp is growing in popularity as a building product. This nonpsychoactive form of the cannabis plant has been utilized for years to make rope, insulation, bioplastics, and other industrial materials due to its strength and rapid growth. Until recently, hemp has remained a peripheral product—the most familiar of which is Hempcrete—in the construction industry. However, it is becoming an increasingly viable option for more common building elements due to developments related to its similarities to—and differences from—wood.

As is now widely appreciated, wood’s carbon sequestration capabilities—and its resulting favorable carbon footprint compared with concrete and steel—have helped lead to its increased use in building construction. However, the resurgence of interest in engineered lumber has raised renewed concerns about overharvesting and deforestation.

Once the U.S. Farm Bill was amended in 2018 to legalize agricultural hemp, entrepreneurs and product manufacturers began to take note. Like wood, hemp stores carbon. However, hemp’s rapid growth makes for a superior carbon-capture feedstock. Hemp can be cultivated in 90 to 120 days, 100 times faster than oak trees. The plant also sequesters four times more carbon than a similarly sized forest. Hemp absorbs more than 20 metric tons [PDF] of carbon per hectare, and its ability to be harvested biannually effectively doubles this quantity. According to GoodEarth Resources, an Australian eco-energy consultancy, “Industrial hemp has been scientifically proven to absorb more CO2 per hectare than any forest or commercial crop and is therefore the ideal carbon sink.”

Engineered hardwood flooring by HempWood
courtesy HempWood Engineered hardwood flooring by HempWood

Many hemp building products utilize the plant’s fibers in somewhat predictable ways, such as for weaving textiles or reinforcing cementitious materials. However, manufacturers are now creating “reverse-engineered” wood alternatives in the form of pressed hemp boards and blocks. In Murray, Ky., Fibonacci manufactures lumber, flooring, cabinets, frames, and furniture with hemp plants harvested within a 100-mile radius for its product, HempWood. The company subjects the plant fibers to high heat before compressing them in molds with a soy-based adhesive, or wood glue. Fibonacci’s aim was to recreate oak’s stability, hardness, and density in a workable, hardwoodlike material. HempWood may be cut, sanded, and finished like oak and is nearly twice as strong, according to its manufacturer. “The whole idea is to take something that’s weak and grows fast, and transform it into a replacement for something that’s strong and grows slow,” HempWood founder Greg Wilson said in a HempBuildMag interview.

Durham, N.C.–based startup Plantd has similar aspirations. Co-founders Josh Dorfman and Huade Tan launched the company after searching for suitable biomass alternatives to trees. They recognized that hemp’s significant strength and carbon performance make it an optimal feedstock for residential building applications. With funding from a North Carolina Idea Seed grant, the manufacturer is prototyping a hemp-based OSB. By creating their own manufacturing processes—which are nimbler, as well as more mobile and economically accessible, than lumber mill machinery—Dorfman and Tan claim that the surrogate material will also be less expensive. According to the Dorfman, a new OSB plant can cost $400 million to construct, while a new Plantd facility costs $1 million. “Our approach eliminates the need for an eight-story tall, 4 million-pound press that the entire industry relies upon,” Dorfman says. “Instead, we build microfactories that are distributed near or in the markets where most houses are being built.” Unlike trees, hemp is a viable crop throughout much of the U.S., enabling a localization of farming and manufacturing that the engineered wood industry cannot attain. Furthermore, hemp can be dried in the same field from which it is harvested and does not require the energy-intensive industrial drying process of green timber.

Plantd hemp-based OSB prototype
courtesy Plantd Plantd hemp-based OSB prototype

Dorfman and Tan are eyeing Elon Musk’s $100 million XPrize for Carbon Removal, a four-year competition that aspires to remove 1 gigaton of atmospheric CO2 annually. In the first stage, the contest requires a demonstration of sequestering 1,000 tons of CO2 per year until 2025, which Dorfman claims can be accomplished with a little less than 100 houses worth of hemp OSB. “Our goals are to reach gigaton scales of CO2 capture, which requires producing materials at the scale of the timber, steel, and concrete industries,” he says.

Ironically, the advantages of modern lumber provide leverage to its newfound competitor. Wood’s newfound popularity is due to its beneficial carbon footprint, renewable growth, and advances in processing—but hemp scores even better in these areas. Additionally, the ability for farmers to participate in an emerging, distributed economy is a plus.

That said, hemp building products may remain confined to shorter spans and interior applications in the near term. HempWood lumber, for example, is currently limited to 6-foot lengths and use indoors. The processing is similar to that of laminated veneer lumber, which was designed to make use of smaller and more varied species of trees and also is largely inadequate for exterior applications.

Plantd's 2x4 hemp prototypes
courtesy Plantd Plantd's 2x4 hemp prototypes

In time, however, hemp will begin to compete with concrete, steel, and wood—particularly in the residential construction arena. This transformation has only been accelerated by the recent scarcity of lumber, leading to escalated prices. After launching its structural panels, Plantd aims to introduce structural framing elements, starting with laminated strand lumber for stud-framing applications. The company will also manufacture hemp I-joists made from OSB and LSL components. “For the housing market, our goal is to eventually offer builders the entire framing package,” Dorfman says. “We're working to introduce all of these products to the market as fast as possible. Our manufacturing technology enables us to produce all of these products from multiple production lines within the same factory, an industry first.”

Hemp-based building products exhibit significant potential structural, economic, and carbon advantages and could represent a disruptive technology in building. Many challenges remain, and such transformation will take time. In theory, if enough new structures are built with hemp instead of concrete, steel, or timber, the construction industry could shift dramatically toward net-zero carbon. “At that point,” Dorfman says, “the construction industry could potentially capture even more carbon than it emits.”

The views and conclusions from this author are not necessarily those of ARCHITECT magazine or of The American Institute of Architects.