“The House That Cannabis Built”

A hempcrete “brick”

Cellulose can be so much more than just trees. One alternative is hemp, which simply by bringing up I am risking being labeled as the class stoner. Despite hemp’s long standing association with a certain illicit substance, it’s usefulness and practicality have long been known to more than hippy jewelers, it is fast growing, versatile, strong, and resilient.  It is, in fact, a weed. One promising application of hemp in construction is “hempcrete” which is a mixture of hemp fibers, a lime binder, and water. It resembles concrete blocks, but does not represent a truly comparable product. It has much lower compressive strength (though considerably higher tensile strength thanks to the fibers) which means it is not used as a structural material, but as a building wrapper it has many advantages. It has an r-value ranging from 3.7-5 per inch1, matching or beating most other wall systems, including fiberglass insulation3. It also tends to be used with a wooden frame sandwiched between two layers of hempcrete, meaning that the studs are fully enclosed, and thus don’t act as thermal bridges. The hempcrete is very air-tight, further aiding with climate control, yet it is also hygroscopic, meaning that it can absorb and release moisture, helping to regulate internal humidity, just as it’s mass serves to regulate internal temperature.2 Though not as strong in compression as concrete, it is a very durable material, expected to last as long as 800 years 4 with one bridge in France built with hemp reinforced concrete still standing after more than 1400 years 6. The greatest advantage of hempcrete is in it’s environmental performance. It is carbon negative 5 because of the CO2 sequestered in the hemp plant as it grows, and then the wall continues to absorb more CO2 out of the air, calcifying the lime and making the hempcrete stronger over time7. It’s low weight as compared to concrete (15% by volume) means that transportation embodied energy is much lower, and it’s other properties mean that it represents an ecofriendly lifecycle. At the end of it’s lifecycle hempcrete continues it’s curious and ecologically responsible nature by being both recyclable, and compostable, crushed up it can serve as fertilizer5 (possibly for the next crop of hemp?)

So, what are the downsides? It’s much lower compressive strength (1/20th that of concrete) means that this is not a structural material, though it is likely capable of supporting it’s own weight. The main problem is the same as with most “green” technologies, cost. In this case however, the inherent cost isn’t much, if any, higher than other wall systems, the problem lies in legislation. In Europe and Canada hemp is grown legally and widely, and though this material is fairly new (in modern times, there are ancient examples as well) it has begun to gain some traction. In the US however, hemp is not distinguished from it’s more potent cousin, and thus is illegal to grow nationwide, though several states have legalized it’s production, the DEA has not agreed to grant exceptions. Until this is changed hempcrete promises to be much more expensive in the U.S. than elsewhere. This is a problem, since beyond hempcrete there are many applications of hemp, as since it is one of the fastest growing crops, and can grow on land which isn’t suitable for many other crops. Furthermore it is important for agriculture in this country to diversify, especially as we are faced with increasing challenges from climate change, resistant pests and diseases, and increased concerns over chemical use in agriculture.









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