Hemp house rises in rural Sebastopol

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.

Broad interest in hemp construction has been mounting for decades. Now that lawmakers have legalized industrial production, some hemp industry analysts predict the U.S. market will hit $20 billion by 2023.

Sheldon said environmentally minded architects and builders have been experimenting with hempcrete for the past 25 to 30 years. Fewer than 100 hempcrete houses have been built in the U.S. For one thing, it has been faster and cheaper to build houses with cement and fiberglass, despite the environmental cost of using those materials.

“Once cement was developed, people started using it exclusively because it's much faster,” he said. “You plaster it with speckled cement, two days you can put the next coat on. People didn't think about carbon.”

Cement is the most commonly used human-made material. When heated, the limestone used to make cement breaks down into calcium oxide and carbon dioxide. This process accounts for about 50 percent of all emissions from cement production, according to data from Columbia University. Making cement accounts for 7 percent of the world's carbon emissions.

Sheldon said the process of getting a permit to build a hempcrete house in Sonoma County was no easy task, especially in the wake of the destructive October 2017 wildfires. Sheldon was able to get information from a developer who built a hempcrete home in Nevada City, and he submitted an application for a building permit. The process was slow, but Sheldon had permits in hand by the beginning of summer 2018.

Sheldon said hempcrete has shown signs of being fire resistant, but cautioned that few studies have been performed in the United States. England, France, Australia and Canada have conducted more extensive testing on hempcrete and found mostly positive results.

“When people read that it's an insulation material, it's fire resistant, there's no mold or rot, that's a good pitch,” said Sheldon. “Plus the energy costs will be lower. We want to get this down to as close to zero net energy as possible.”

Sheldon said he hopes to have the house finished by early summer, just about a year after construction began. He is still not certain how long the house will take to dry after a period of heavy rain, but said the crew has a deadline to finish plastering by the time a flooring specialist arrives in April.

Drying time varies by the product and the climate, but on average it would take one to two months before the moisture is down to an acceptable level.

“There are a few things that we don't know but will learn, like how long (hempcrete) takes to dry,” said Sheldon. “You can think it through as much as you want, but when you have a mixer out there and start putting it together it's a different thing.

He pointed to a California code requiring all new homes to be zero net energy by 2020. That means energy consumed in a house will have to be less than or equal to the on-site renewable energy. An architect who started his career in the 1970s, Sheldon said he has witnessed a fundamental shift in perspective on energy and climate change.

“We don't have much time to get things right now,” he said. “It's a crunch.”