Cannabis habitats: Hemp as a building aid comes of age
Tuesday, January 09, 2018
Despite the recent Justice Department ruling, the forward march continues toward pervasive legalization of the plant cannabis sativa.
A survey of U.S. state marijuana laws shows that, as of 2018, a decisive majority of states have legalized medicinal use of marijuana — and some even its recreational use (in seven states and the District of Columbia). Not surprisingly, legal cannabis businesses are expanding and multiplying across America.
That goes for businesses that would exploit hemp, the nonrecreational, nonmedicinal variety of the cannabis plant (although "hemp" often doubles as the common name for plants of the entire genus cannabis).
Industrial hemp is used, among other things, as a superior but plentiful and renewable building material. It turns out legalization is managing to popularize and legitimize this noncontroversial, nonpsychoactive variety of cannabis sativa, along with its uses for construction (mostly as an insulator).
Among the best-known products derived from industrial hemp is hempcrete (called as much because it's a mixture, like concrete). It's touted and repeatedly given trials by leading green-building websites, such as greenhome.com and greenbuildingcanada.ca. The latter published a recent study that expansively addresses not just hempcrete's historic evolution but its numerous advantages (at least six major ones and counting).
Hempcrete is a so-called biocomposite of the stalk elements hemp hurd or shiv and a mineral binder (lime), which fuse in a process known as "bonded cellulose insulation." The binder, once moistened and setting, coats all of the shiv small pieces and binds and glues the hurd particles. Once the binder is cured and the admixture dries, the hempcrete is done.
In appreciating hempcrete's applications, one must know how it's different from other construction composites, such as concrete, mortar and plaster. The lime binder does not fill all hemp particle voids. It just causes the particles to stick with one another where they happen to touch. That's why hempcrete in the final mixture usually has a high percentage of void space between its other components.
But the void space is fine since hempcrete is intended as both a structural and insulating material. Its wide range of helpful thermal, structural and drying properties enables the insulating function. It can be used as roof, slab or wall insulation, although each use requires a slightly different ratio of components in the final biocomposite mixture.
The significance of hempcrete as an insulator? For years, the building industry has been adding more insulation to buildings to lower their energy use requirements. Insulation keeps the air space inside a building at a more even temperature. That means less work for any HVAC system.
Other insulating products aren't just expensive to use; some have serious environmental or health impacts. Going forward, the building industry is using an increasing amount of conventional insulating material — such as mineral wool batt, fiberglass batt, denim batt, dense-packed cellulose, extruded polystyrene foam or expanded polystyrene foam — which is making the cost of hempcrete, albeit a plant-fiber choice, already comparable.
And then hempcrete brings its own advantages. Hempcrete can maintain its integrity under the humid conditions common in many parts of the world (even more so than other plant-fiber choices like cellulose, wood fiber, straw bale, straw/clay and cotton).
Hempcrete takes on moisture when it's present. It then releases it when conditions allow — even in areas where there is sustained relative humidity of 93 percent. The lime binder's high pH is antimicrobial and antifungal. Thus, the coating around the hemp hurd portion resists the development of mold even better than that of other insulation materials.
This resilience in the face of moisture — whether as high humidity or liquid — makes hempcrete unique among building insulators, and allows it to serve well in both hot and cold climates.
As for hempcrete's structural role, it admittedly has a minor one. Then again, batt, loose fill and spray insulation materials have no structural role. Builders know that if hempcrete is cast around conventional wall framing or double-stud framing, it helps prevent them from any bending or buckling under load. In addition, its rigidity and textured surface make it an excellent substrate for new finishes.
Finally, hemp is plentiful. It's a high-yield agricultural crop. "In terms of plant biomass available for use from a single crop, no other plant provides as much volume as hemp," say the authors of the Canadian study.
Whether the hemp plant is being grown for its strong fiber or the seed it produces, the hurd is always a plentiful byproduct — especially because the hurd also figures in supplying markets for animal bedding and fuel pellets. So really any large-scale hemp production usefully generates tons of hurd for the building market.
But perhaps what can't be overstated is the importance of hempcrete's moisture-handling abilities. It doesn't just mean that as an insulator, hempcrete will be mold-free. Those abilities also reduce the chance that a poor indoor environment presents itself at all, due to any excessively wet or dry air lingering in or around the building.
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