Hempcrete: How Sustainable Construction Could Change the World

European building projects have relied on hempcrete to construct insulating, non-weight bearing infill walls since the 1990s. In fact, the popularity of building with hempcrete grows year over year in France but, in the United States, a builder must acquire a special permit to work with the materials.

In Bellingham, Washington, one homeowner took on a project using hemp-lime and weighed in on the pros and cons of building a house with hempcrete. Pamela Bosch is in the process of renovating her 1960’s home using hemp-lime and has named the project the Highland Hemp House. She credits an intrigued Planning and Permit department worker and a town interested in sustainability for the ease of getting the proper permits for building. In fact, because the Highland Hemp House design was energy-efficient, Bosch’s permits were fast-tracked.

However, the process still wasn’t simple. Finding lawyers, builders, and other necessary ancillary contractors to work on a hempcrete project led to unexpected costs, much of which was due to a lack of understanding. Builders don’t know how to work with it, insurers don’t know how to quantify it, lenders can’t estimate it — the list goes on and on. Ignorance and unfamiliarity with hempcrete have led to disinterest from American building projects. It has also made things more difficult for private citizens who are interested in using the material in their own renovations or projects.

What is Hempcrete?

Hempcrete is a biocomposite building material made by mixing the woody inner lining of the hemp stalk (hemp hurds) with either lime, sand, or pozzolans. It can be found marketed under names like hemp-lime, canobiote, canosmose, and isochanvre. Many builders find hempcrete easier to work with than other lime mixes as it is not as brittle as concrete, and that is the first of many differences between the two building materials. Hempcrete also has just 5% of the compressive strength of residential concrete and 15% of the density of traditional concrete. Because of this, hempcrete walls must be supported by a frame made using a different material. This grants the benefits of hempcrete while still supporting a vertical load.

What Are the Advantages of Hempcrete?

There are many advantages to building a structure out of hempcrete. The composite building material acts as an insulator while also regulating moisture and humidity. The porous structure of the materials and the alkaline property of the lime contribute to this effect. It can also absorb excess moisture from the air when humid to cut back on the risk of mold spore formations.

Some reports also suggest that hempcrete could be fire-resistant, and it is a certified fire-proof building material in France and the UK. However, metrics system-based tests and data have been widely ignored in the States. Many building inspectors hesitate to grant permits to build the lime composite material. Sources also state that hempcrete is pest resistant, though the source data for these claims haven’t been located.

One very attractive advantage of building with hempcrete is how sustainable the material is — it is carbon-negative. In fact, during the growth of the plant, hemp will absorb so much carbon that more CO2 is absorbed in a hempcrete wall than is used in the entire process to build it. It can also work well combined with original materials when fortifying historic buildings. And as it ages it will turn to petrified rock, so in the best case, the structure could last hundreds of years.

What Are the Disadvantages?

There are some real challenges to working with hempcrete. It has about 1/20 the strength of concrete and, because of this, a structure built with hempcrete requires a timber frame made of steel or concrete to support the load of the house. This is one place where Pamela Bosch and her Highland Hemp House hit trouble. Her town of Bellingham is in a challenging seismic zone for building, and hempcrete is not yet ICC (International Code Council) ASTM (the American Society for Testing and Materials) certified. To compensate, Bosch had a stand-alone internal framework engineered, an expensive setback.

The hemp advocate also had to finance the project herself and take on all liability with the city, and high startup costs are very common when building a hemp house. Without any prior experience with hemp structures, banks and insurance companies would not support the Bellingham building. Complications due to inexperience are also common in the engineering and building process. Some hemp projects have hit major complications when builders treated the material like traditional concrete. With how rarely hempcrete has been used in construction projects, there really isn’t much experience to pull from.

Where do you get the hemp?

Hemp production has picked up in the United States, but many of the plants are grown to be manufactured into CBD oil. These plants are generally more knobby and less ruddy, which isn’t ideal for making hemp hurds. Even if farmers got on board growing different varieties of hemp, there isn’t any infrastructure in place to process the materials. For the Highland Hemp House, Bosch shipped a 40’ container of hemp hurd in from the Netherlands, which doubled the cost of the materials.

She explained that despite the doubled cost, it was a small fraction of the building process.

“Of more consequence was the time, effort, and expense of breaking with the status quo,” Bosch said. “Contractors, engineers, plumbers, electricians, city attorneys, insurance companies, (and the DEA), generally had to be persuaded that it would not be too painful to try something new. The braver and more philosophically inspired were among those I was able to contract, but the novelty also generally accrued a fee.”

Some will look at the positives and negatives of building with hempcrete and agree that the building material is worth exploring, but what will it take for the United States to accept hempcrete? More research. Bosch’s project makes it clear that bias is one of the biggest hurdles in projects like the Highland Hemp House, and supportive research is the best way to sway bias. American architects and engineers are currently only taught how to work with wood, concrete, and steel. Research-supported data could shake up the curriculum and drive more brilliant minds to better figure out hempcrete construction projects.