Tucked away on a quiet country lane in Sebastopol, a construction crew is building a simple house with sturdy walls and large windows. The only unusual thing about the project is hidden inside the walls.
The house is being made with hempcrete, a non-load bearing material made of industrial hemp.
“I became familiar with hempcrete because I’m always looking to build in the ‘greenest,’ most sustainable way possible,” said Steve Sheldon, an architect, property developer and builder. “We’re always looking for materials that are rapidly renewable, and as carbon-neutral as possible.”
For more than four decades, Sheldon has designed homes and buildings with sustainable building materials and energy-efficient features. When a client expressed interest in building a “unique” accessory dwelling unit, Sheldon said his mind jumped to a newer version of one of the world’s oldest building materials.
Hempcrete is made by mixing the wood-like core of the hemp plant with hydraulic lime and water. The result is a sturdy, breathable building material with a lower carbon footprint than concrete and a host of other benefits.
At the construction site in Sebastopol, a work crew formed three blocks of hempcrete, each configured for a different purpose. The lightest block represents the material poured for the roof of this 1,000-square-foot house. Harder-packed hempcrete forms the walls and floor of the structure. Later, these walls will be coated with more lime to form a smooth surface.
Mike Brandt, a construction supervisor, said he was initially skeptical about the material.
“It took me a while, but now I’m on board with (hempcrete),” he said.
What are the downsides? Sheldon said hempcrete costs more than traditional cement and takes longer to cure. He declined to disclose his cost. But various online sources seem to indicate it could cost 10 to 20 percent more than conventional construction.
While there are guidelines for mixing it, Sheldon said his team did a fair amount of testing before discovering the right density for each section of the house.
At this point it’s essentially a mix-it-yourself process.
Hempcrete also has a long list of benefits, said Sheldon, ranging from energy savings to mold and pest protection, fire resistance, temperature regulation and sustainability. Hemp is a rapidly renewable building material, growing from seed in about four months.
“When you build a straw-bale house, you lock that carbon into the building,” said Sheldon. “Same idea with hemp.”
Walking a few dozen feet from the home site, Sheldon swept aside a tarp to reveal a large pile of chopped hemp stalk, which looks like a normal pile of wood chips or mulch. When formed the blocks look like brownish-gray, fibrous cinder blocks. But they weight eight times less than concrete.
The hemp was grown in Kentucky, a state that trails only Colorado in production. With passage of the 2018 Farm Bill, lifting industrial hemp’s designation as a controlled substance, farming of the product is now free to expand into other states, including California.
Hemp is a mild-mannered member of the Cannabis sativa species, similar to cannabis in form but lacking most of the tetrahydrocannabinol (THC) that produces a psychoactive effect. Known for producing durable plant fiber, hemp has long been used to make rope, paper, oils, clothing, and, more recently, insulation material.
