On the Cape Cod beachfront, construction with industrial hemp took a major step forward this summer after 80 years of prohibition with the Cape Cod Hemp House, in Harwich, Mass. The new construction is a 6,000-sq. ft. net-zero energy use home, insulated with “hempcrete,” a mixture of Cannabis sativa stalk material and a more typical limestone-based binder.
The house is a pioneering structure in the United States, the owners say, engineered to survive climate-related sea level rise and avoiding almost all petroleum-based building materials, which slashes its carbon footprint. Natural insulation reduces energy use and improves the indoor environment, they say.
But its use of industrial-scale equipment to blow in hemp-based insulation — which helped them build walls in record time — meant builders had some ’splaining to do: The wind scattered woody hemp material and lime dust into the neighbor’s yard, said Zachary Miller of CH Newton Builders.
“The neighbor complained when the hemp blew all over his landscaping,” Miller told The News Station. “He assumed the hemp was poisonous because it was insulation. I had to explain that it was non-toxic and natural and it’s even used for mulch in Europe,” he added of the continent, which has been using industrial hemp for nearly three decades.
Miller’s company specializes in “complex and high-end residential projects” in Rhode Island and Massachusetts but this was their first build using hempcrete, he said.
The fire resistance and insulation properties of hemp and lime were intriguing to local permitting officials, and the local fire marshall visited regularly to check out the project, Miller said.
The luxury home wasn’t, however, cheap. Building during a pandemic, importing hemp hurd and binder, and paying travel expenses for the designer of the spray rig, AKTA’s Laurent Goudet, to come from France drove the price up and expanded the timeline of the project, owners said.
A new type of wall
The hemp-based insulation material dries to a fibrous thick wall enveloping the 2” x 4” studs, replacing traditional wall assemblies that attempt to block water migration with a flawed “five-layer dip” approach of sheetrock, insulation, plywood, Tyvek plastic wrap and siding. The fiber wall is then plastered over on both sides.
Sealing traditional walls often traps-in mold and rot, causing unhealthy indoor air quality in traditionally built homes that can lead to Sick Building Syndrome in the people who live there. The Environmental Protection Agency first identified SBS in the 1990s.
Hemp and lime insulation, however, is vapor-permeable, which means the walls regulate humidity via a capillary motion through the hemp strands and lime.
“As we started to discuss the R-value and the vapor migration through the hempcrete, it occurred to me that wow, this is a magical insulation property that everyone is constantly imagining as impossible,” Miller said. (R-value refers to a material’s insulating capacity.)
Hempcrete, however, is not load-bearing, so it has to be applied around framing elements of timber or steel. This means builders need to plan around a new schedule for rough electrical and mechanical inspections before the outer walls are built.
Fighting climate change
But hemp’s real superpower as a building material is the plant’s amazing capacity to act as a carbon reservoir, and the material’s ability to maintain that capacity. Fast-growing, renewable hemp stalks absorb around 3-6 tons of carbon dioxide per acre through photosynthesis while growing, about three times more than trees (depending on their age). Hemp continues to hold onto that carbon dioxide for years in the walls of a hempcrete building.
Using this eco-friendly plant counteracts the construction industry’s destructive effects on the planet’s climate. The building industry spews out about 40% of the total greenhouse gasses in the atmosphere and construction waste makes up about 25% of the earth’s landfills.
The Biden administration has pledged to retrofit and build millions of homes to fight climate change, so green builders believe that bio-based building materials like hempcrete could break through widely in the United States in the next decade.
Hemp was illegal to grow in the United States for 80 years after federal prohibition of marijuana in the 1930s, sweeping up hardworking, non-intoxicating industrial hemp into the Drug Enforcement Agency’s Schedule 1 prohibition of cannabis products. Industrial hemp wasn’t re-legalized until the 2018 Farm Bill.
“When we look at France, industrial hemp was never outlawed,” Kiko Thébaud, a Boston-based architect and the project’s hempcrete consultant, told The News Station. “France and China have very stringent rules about marijuana, but they have a tradition of growing industrial hemp and an understanding of the plant’s benefits for textiles, rope, paper and building materials.”
The United States had a “topsy turvy relationship with hemp in the 20th Century, associating it with pot,” and losing out on the benefits of industrial hemp that are only now being slowly achieved, Thébaud said.
While the Cape Cod house will be covered on the outside with cladding to match the neighboring homes, the inside will be lime-plaster walls, which have superior acoustic and insulating properties, Thébaud said. That is similar to older brownstone homes in Boston’s Back Bay area.
“When you walk into a lime-plaster room and experience it, it just feels different,” he said.
“My next house is going to be a hemp house,” Thébaud added. “I could never go back to fiberglass or foam insulation.”
“After 40 years, that batt mineral wool ends up a big mess and goes into a landfill,” he explained. “How many landfills are we going to keep filling?”
Looking to the future
But building one dream house at a time isn’t the way to allow carbon-sequestering hemp to make a difference, the Cape Cod owners believe.
The industry needs a boost to take hemp construction to the next level, homeowner Michael Monteiro (a former CEO of the property management software app Buildium) told The News Station.
This spring, Monteiro started a hemp-building incubator, Mpactful Ventures, to promote industrial use of hemp lime insulation and push the industry forward in a significant way. He wants others to use the industrial spray rig from France, which his project employed, to build larger and more complex projects.
“We want to de-risk the use of [the equipment] and inspire others to be bolder and think about what can be done,” Monteiro said.
Already, Monteiro’s builder Miller said one of the obvious applications for the spray machine could be historic renovations. In Europe, half of hempcrete projects are renovated properties.
New England cities like Providence, R.I., have “reclaimed old beautiful brick mill buildings with big open windows and timber frames,” he said. But the interior structures are “a nightmare” to heat and cool, he said. Currently, renovators pump in foam or install batt insulation and then cover the existing walls with sheetrock, destroying the feel of the building, Miller said.
But spraying them instead with a 3- or 4-inch layer of hempcrete insulation and covering them with lime plaster would keep the feeling of a thick masonry element while adding thermal insulation properties, he said.
“This would be a perfect early adoptive use,” Miller added.
“Hemp is not new and has been in use for many decades elsewhere,” Monteiro, the Cape Cod house’s owner, said. “Hemp can be part of the solution. It’s a healthy material that can change the carbon footprint of a house to build it to high-performing standards.”