As more states approve the medicinal and recreational use of marijuana, the building industry is taking note. A strain of the Cannabis sativa plant, hemp is a promising material for buildings that has been similarly regulated, though it differs in several ways from the everyday variety. The presence of buzz-inducing tetrahydrocannabinol in industrial hemp is quite low at around 0.3 percent, as compared to the 20 percent of recreational cannabis. Hemp must also be grown outdoors, can reach up to 15 feet tall, and has a dense, fibrous core suitable for making rope and other fiber-based substances.
In fact, hemp has a history in construction. Roman engineers used its fibers to enhance the mortar for bridge abutments, for example. Because the plant grows rapidly, without the need for pesticides or chemical fertilization, and improves soil health with air-circulating roots, it is an attractive crop for farmers. Although special permits are still required to use hemp-based building products in the U.S., a variety of potential applications suggests a promising future for the plant.
The most familiar example is Hempcrete, a lightweight, cementitious composite made with industrial hemp hurds (woody fibers from the plant core), lime, and water that can come in modular blocks similar to concrete masonry units. Hempcrete should not be mistaken for reinforced concrete, as the materials is not structural, nor can it be used in foundations since the natural fibers will degrade with prolonged exposure to moisture.
Instead, the product is suitable for constructing monolithic, above-grade walls. According to Winston-Salem, N.C.–based distributor American Hemp, Hempcrete replaces the functionality of OSB, insulation, and drywall. No cavity is required although structural framing is necessary. Given its ideal application in low-rise construction and its stucco-esque appearance, Hempcrete is suitable for detached single-family residences. According to The New York Times, the first modern hemp residence was built in 2010, and some 50 houses now exist in the U.S.
For architects and clients seeking a structural material, Calgary, Canada–based JustBioFiber offers a hemp-based modular block for load-bearing wall construction. The Super SSR Block exhibits a compressive strength of more than 5,800 psi, which compares favorably to the range of 1740 psi to 3045 psi typical for CMUs. Additionally, the product is an excellent insulator and is highly fire-resistant with over a one-hour rating. Super SSR Block also has high thermal mass, can regulate temperature and humidity, is breathable, and resists mold and pests.
Of particular note is the product’s advantageous environmental footprint. JustBioFiber reports that its product sequesters 243 to 287 pounds of carbon dioxide per 1.3 cubic yards, based on the quantity of carbon-storing plant fiber used. Additionally, the blocks recarbonate an additional 141 pounds per 1.3 cubic yards during the curing process. Considering that the primary production of conventional concrete has a footprint of 456 to 689 pounds of carbon dioxide per 1.3 cubic yards, Super SSR Block has favorable environmental performance. The company claims that the production of 4 million blocks emits 7,165 tons of carbon dioxide, which is equivalent to 115 pounds of carbon dioxide for 1.3 cubic yards of block, or 0.17 to 0.25 that of concrete. The additional 243 to 287 pounds of carbon dioxide per 1.3 cubic yards stored in hemp fiber—which is about four times that of trees—results in a product that acts as a carbon sink rather than a carbon emitter. This “carbon less-than-zero” product, as JustBioFiber calls it, also enhances the energy efficiency of buildings with its high insulating capacity—resulting in a 64 percent reduction of greenhouse gas emissions compared with building code standards.
Hemp is certainly not limited to concrete. The material may be readily fashioned into boards, batts, molded composites, textiles, and more. HempBoard, for example, is a replacement for particleboard, with variants that meet or exceed ANSI specifications. The material functions well for doors, millwork, and furniture, with different versions for interior and exterior applications.Thermo-Hemp is batt insulation that exhibits excellent thermal and acoustic properties. Available in thicknesses from 1.1 to 9.5 inches, the product is pollutant-free, biodegradable, and fire-resistant. Like other hemp-based products, Thermo-Hemp is carbon sequestering and absorbs 28.7 pounds of carbon dioxide per 35 cubic feet of material.
Hemp fiber may also be used to make moldable composite materials with thermoplastics, such as polypropylene and polyethylene, or bioplastic alternatives. Potential applications include furniture, countertops, and injection-molded consumer products. It is also suited for making textiles due to its high tensile strength, breathability, and UV resistance. According to Simplifi Fabric, a Canadian purveyor of eco-friendly materials, hemp’s advantages include the fact that its crop yields much more fiber per area than either flax or cotton. As a textile, hemp is similar in feel to linen or flannel.
Yet another hemp application is finishing oil, such as tung oil or varnish. Made from pressed seeds, hemp oil saturates and seals wood, protecting it against moisture and physical abuse in both interior and exterior environments. Unlike polyurethane coatings, hemp oil adjusts to the natural expansion and contraction cycles of wood and releases any trapped moisture.
So if hemp is such a super-material, as many call it, why is it not more common? As mentioned above, regulation is the primary obstacle. Although hemp seeds are now available in the U.S., farmers still require permits to grow the plants. Furthermore, the requirement of building code exceptions for hemp-based construction serves as another roadblock. Nevertheless, the combination of clear economic and ecological advantages for industrial hemp may eventually lead to a breakthrough. One possibility is the passage of the Industrial Hemp Farming Act of 2017, which, if successful, will eliminate the controlled substance designation for hemp. Although the chance of the bill's passing is uncertain, it would bring about what The New York Times characterizes as “an Olympic leap toward a burgeoning agro-business.”