The walls in Gregory Ferembach’s public housing building are lined with one of nature’s best insulation materials: hemp. “We’re never cold in winter,” Ferembach said in French. “The kids walk around barefoot all the time, or even in their underwear.”

Grist / Amelia Bates

It helps that their apartment is on a middle floor, and their building is sandwiched between two others, but the coziness also owes to the “hempcrete” in their walls. This is  a concrete-like blend made by mixing hemp hurd — the woody core of the cannabis plant — with water and lime.

John Washington says sustainability is “not just saying we want homes that have solar panels and that require less fossil fuel in the heating system,” he said. “What about the fossil fuel, chemical, and plastics production in the process of building and rehabilitating the home?” Conventional building materials pollute our indoor air, too: many of them release volatile organic compounds (VOCs) like formaldehyde, a known carcinogen, and other harmful chemicals. That’s without counting materials like lead and asbestos, which continue to plague public health even decades after they were banned.

“In Buffalo and in many Rust Belt cities,” Washington said, “we have folks whose homes literally are killing them.” The effort to retrofit those homes as cheaply and efficiently as possible usually leads contractors back to petrochemicals.

Hempcrete has gained a dedicated following among green builders, featuring in homes from Cape Town, South Africa, to Cambridgeshire, U.K. to Asheville, North Carolina. With the Biden team promising to retrofit and build millions of homes to fight the climate crisis, sustainable materials like hempcrete are poised for a much wider breakthrough in the U.S.

Fabrice MALZIEU/ NXNW / LMi

As construction scales up, the question is: can hemp be part of an affordable housing future — not just one for eco-conscious elites? Paris is determined to show that it can be. In pure insulation value for a given thickness, it’s slightly less effective than typical, plastic-based materials like fiberglass or foam board.

Hempcrete acts like a “sponge,” said Benoît Savourat, a French farmer and the president since 1998 of the influential hemp growers’ cooperative La Chanvrière de l’Aube. It absorbs moisture from the surrounding air when it’s humid and releases it again when it’s too dry. That plays a crucial role in stabilizing the “felt temperature” of a room. “When you’re in a humid environment, you need to heat it to 70 degrees or more,” said Savourat. “With hempcrete, you’re perfectly comfortable at 65.” Conversely, summer air doesn’t feel as sticky, making it tolerable without air conditioning. That means less energy use year-round, and could make a big difference as summer temperatures continue to rise.

Hemp’s real edge when it comes to fighting climate change, though, is in the carbon it absorbs throughout its life cycle. In a growing season of about eight weeks, an acre of the crop can sequester  more than a typical acre of trees can absorb in a year. Different parts of the plant can then go to a variety of uses, including its strong outer fibers, which can be woven into textiles, pulped into paper, or even transformed into forms of plastic. That leaves the inner core, or hurd, which is the main ingredient of hempcrete. Combine the hurd with lime, a powder derived from limestone which acts as a binder, and you get the sustainable building material.

Once blended together, hemp and lime continue to absorb carbon, offsetting emissions from other construction materials. Altogether it has been found that building a wall with hempcrete generates roughly three times fewer carbon emissions than building a wall with conventional methods.

Ferembach’s building, on Paris’ Rue Oberkampf, was designed by architects Barrault Pressacco and built in 2017. Clément Guillaume / Metropolis Communication

Paris Habitat inaugurated the city’s first hempcrete development in 2012. The five-story building on Rue Bourgon is tiny by social housing standards, with just eight apartments. But even today, it remains an outlier in the single-family-dominated world of hemp construction. Paris has meanwhile added at least three more hempcrete buildings, totaling about 40 subsidized units, and more are in the works.

Hemp and other natural materials still represent a small fraction of overall construction in Paris, including social housing. But Quet Hamon says they are becoming more mainstream in renovations, and Paris Habitat plans to use such materials in at least half of the thousands of social housing units that will need upgrades in the coming years.

You can read the original article at grist.org


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