Green Building Materials: Hemp and Hempcrete
Hemp has been around for centuries. It has been farmed in China for 5000 years, and George Washington, Thomas Jefferson, Ben Franklin and Henry Ford grew fields of it as an agricultural crop. It was the source of most everyday items, like rope, lighting oil, paper, fuel, paints, medicine, food, lumber and animal bedding. It was the raw material for all textiles until the 1820s and the invention of the cotton gin. Despite that, throughout the 1800s, hemp was the largest cash crop in the US.
In the 1930s, hemp was predicted to be a billion dollar crop in the US, and all paper was going to be made of hemp by the ’40s to save the forests. In 1937, though, Pierre DuPont, Andrew Mellon and William Randolph Hearst lobbied and pressured Congress to outlaw it. Hearst was in timber and publishing, DuPont and Mellon were in oil, and Mellon provided financial backing. They pushed to replace hemp with petroleum based products and trees, and they succeeded.
Durable, sustainable hemp
Hemp is an amazingly versatile, durable and renewable plant. It is the second fastest growing plant behind bamboo. It grows so fast and thick, weeds don’t have a chance, so it needs no pesticides. It is also highly adaptable to most climates and is drought tolerant.
A crop is mature in less than four months for more than one harvest a year. The entire plant can be used, which you can’t say about many crops. The inner core is used for building, the outer core is used for textiles, and the seeds are used for food, paints and fuel.
Hemp replenishes the soil with nutrients and nitrogen, making it a good crop to rotate with rye or barley. Hemp is superior to trees, because it sequesters four times as much carbon as they do, and an acre of hemp produces four times as much pulp as four acres of trees. Trees take 20 years to mature for lumber, and hemp takes four months.
It was outlawed in the US in the name of greed and power, not for practical reasons. Hemp is legally grown in the UK, Canada and Europe, but it can be shipped to the US for building use. If the US would legalize it, the economy could see some recovery!
What is hempcrete?
Hempcrete is the inner part of the stalk, or the shiv or hurd, mixed with lime and water. Hemp has the unique quality of being able to bind with lime the way no other plant can. It is mixed and filled into temporary forms within a frame, similar to rammed earth. Because hempcrete has high insulating properties, the wall is complete and does not need further insulation. Alternatively, manufactured blocks are placed around the outside of the frame, much like bricks.
Hempcrete is 1/7th the weight of concrete, making it easy to work with and move around a work site. It is not strong enough to be a bearing wall, but it can add to the racking strength of a building.
Acoustic qualities and fire and termite resistant make hempcrete even more attractive. It is also breathable, so it regulates temperature and prevents moisture, mold and mildew from building up. This improves indoor air quality and the comfort of the interior space, which reduces heating and cooling bills
Since hemp is such a low-carbon product, a home will have a very low embodied energy. The fact that it needs to be shipped to the US from other countries keeps it from being carbon negative here.
Uses for hempcrete
Hempcrete is not just for walls. Fiberboard can be used for insulation in floors, the roof and on the exterior of the building. It can also be an insulating slab. There is not a place a hempcrete product cannot be used in a home!
I have wanted to build a cob house since I wrote this piece, but I think hempcrete is my new favorite building material! As always, check your building codes and neighborhood covenants before starting any building project. Enjoy!
Here is more information on hemp and hempcrete from the US suppliers: