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House of hemp? Pushing cannabis as a construction material

March 24, 2012 | 10:53 am

Hempcrete house Push DesignWoody Harrelson championed the environmental benefits of hemp. Giorgio Armani and Calvin Klein incorporated it into their collections. Now a company promoting hemp as the eco-building material of the moment said it wants to build California's first hemp house.

Knapp's Castle siteHemp Technologies said it wants to use hemp-based materials to construct a 500-square-foot structure at the ruins of Knapp's Castle near Santa Barbara. The castle, completed in 1920, was built for Union Carbide founder George Owen Knapp but destroyed by wildfire in 1940. Since then, all that has remained on the property are the sandstone blocks outlining the once-grand estate.

The principal material for the project is Hempcrete, made of the woody internal stem of the Cannabis sativa plant, which is processed into chips and mixed with a lime-based binder. That concoction is then sprayed on, poured into slabs or formed into blocks like concrete to create the shell of a building. Interior surfaces are plastered, and exterior surfaces are stuccoed.

“The walls are to be framed and earthquake-braced internally with lumber,” said Greg Flavall, Hemp Technologies' co-founder, who added that “hemp is very close in cellulosic value to wood.” The material helps to keep structures warm in winter and cool in summer, he said.

Other advantages, advocates have said, beyond the fact that the plant can be grown with little water and few pesticides: Hemp is resistant to fire damage and termites. Although air does not pass through the walls, moisture naturally dries out, and mold or dry rot are not problems. David Madera, who co-founded Asheville, N.C.-based Hemp Technologies with Flavall, credited the walls' lime content, which needs carbon dioxide to harden.

The lime will pull carbon dioxide from the environment after the home is built “because the lime wants to go back to being a rock,” Madera said. “That means the wall is going to get harder and harder over time.”

Flavall said the Knapp's Castle structure “will lock in about 12 tons of carbon dioxide that would otherwise have escaped into the atmosphere when the hemp cellulose decays in the field.”

That is, if the structure is completed. A Santa Barbara County official said a project has been submitted for planning approval but still needs to pass most stages of the permitting process. A house, outbuilding or any other structure would have to be deemed appropriate for the site, and any unconventional materials would have to be declared safe.

Madera, who declined to disclose the identity of his firm's California client, said Hemp Technologies has consulted on three Hempcrete houses in North Carolina, where the material was treated as an alternative form of insulation.

Although hemp contains only trace elements of tetrahydrocannabinol, or THC, the active ingredient in marijuana, hemp is still derived from a cannabis plant. State and federal laws do not make a distinction between varieties of cannabis used for clothing or building materials and varieties used for more psychoactive purposes, so producing hemp is prohibited. Importing hemp, however, is legal, and various companies are pushing forward with hemp building products too.

CannabricHemp Traders in L.A. distributes Hemp Board, a medium-density fiberboard (MDF) made from the same woody material that goes into Hempcrete. Hemp Board President Lawrence Serbin said his MDF is a sustainable alternative for doors, cabinets and shelves.

Spanish architect Monika Brümmer has developed Cannabric and Cannapanel, which, as the names suggest, are cannabis-based building blocks and wall panels. She has completed several residential projects in her country, where the products met construction standards.

-- Jeffrey Head

Photos, from top: Another Hempcrete house in North Carolina. Credit: Push Design. The site of Knapp's Castle in Santa Barbara County. Credit: Pfly / Wikipedia Commons

Corrected: An earlier version of this post ended with a photo of another home identified as one of three hemp houses in North Carolina. Though initial plans called for that house to use hemp, it ultimately was built with different materials, so we have deleted the photo.

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