Hemp is turning a new leaf. The plant fiber, used to make the sails that took Christopher Columbus' ships to the New World, is now a building material.

In Asheville, N.C., a home built with thick hemp walls was completed this summer and two more are in the works.

Dozens of hemp homes have been built in Europe in the past two decades, but they're new to the United States, says David Madera, co-founder of Hemp Technologies, a company that supplied the mixture of ground-up hemp stalks, lime and water.

The industrial hemp is imported because it cannot be grown legally in this country — it comes from the same plant as marijuana.

Its new use reflects an increasing effort to make U.S. homes not only energy-efficient but also healthier. Madera and other proponents say hemp-filled walls are non-toxic, mildew-resistant, pest-free and flame-resistant.

"There is a growing interest in less toxic building materials, says Peter Ashley, director of the U.S. Department of Housing and Urban Development's Office of Healthy Homes and Lead Hazard Control.

"The potential health benefits are significant," he says, citing a recent study of a Seattle public housing complex that saw residents' health improve after their homes got a green makeover.

The U.S. government has not taken a "systemic approach" to studying chemicals in homes and instead addresses problems such as asbestos, lead, arsenic and formaldehyde only after people get sick, says Rebecca Morley, executive director of the National Center for Healthy Housing, a private research group.

She says green building so far has focused mostly on the environment, not the health of the people inside.

Ashley agrees that federal attention has been "sporadic," but says an interagency group began meeting last year to tackle the issue more broadly. He says HUD is funding more research on the health and environmental benefits of eco-friendly homes.

Some green-rating programs, such as the one run by the private U.S. Green Building Council, give points for indoor air quality.

"We are taking the next step in green building," says Anthony Brenner, a home designer with Push Design who created Asheville's first hemp home. "We're trying to develop a system that's more health-based."

Brenner says he's been searching for non-toxic materials because he wants to build a home for his 9-year-old daughter, Bailey, who has a rare genetic disorder that makes her extremely sensitive to chemicals. "We have to keep her away from anything synthetic," he says, or she'll have seizures.

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He says a hemp home can be affordable, even though importing hemp makes it more expensive than other building materials, because skilled labor is unnecessary and hemp is so strong that less lumber is needed.

The hemp mixture — typically four parts ground-up hemp to one part lime and one part water — is placed inside 2-foot-by-4-foot wall forms. Once it sets, the forms are removed. Although it hardens to a concrete-like form, wood framing is used for structural support.

"This is like a living, breathing wall," Madera says. Hemp absorbs carbon dioxide and puts nitrogen into the soil, so it's good for the environment, he says.

Alex Wilson, executive editor of Environmental Building News, says hemp can be grown with minimal use of chemicals and water. He says it has a midlevel insulating value (R-2 per inch) but is usually installed in a thick enough wall system to make it appropriate for all but the most severe climates.

The mixture, "Tradical Hemcrete," has not previously been used in U.S. homes, but in 2008 it went into a community center on the Pine Ridge Reservation in Badlands, S.D., as well as a small chapel and pottery studio near Houston, says Mario Machnicki, managing director of American Lime Technology, a Chicago company that imports hemp from the United Kingdom.

Asheville's second hemp home will be finished in about six weeks, says builder Clarke Snell of the Nauhaus Institute, a non-profit group of designers, engineers, developers and others interested in sustainable urban living.

Snell says the home, which has 16-inch-thick walls, is airtight and energy-efficient. He expects it to meet rigorous Passive House Institute standards, which call for homes to use up to 90% less energy than regular ones.

"On the coldest day in winter, the body heat of 10 people should heat the home," he says. "We're basically building a European home."

Snell says his group will own the 1,750-square-foot house, and its engineer will live there for a couple of years to monitor energy use.

He doesn't know how much it will cost because, as a prototype, it was built with donations and volunteer labor.

The owners of the first hemp home say it cost $133 a square foot to build, not including land and excavation.

"That's pretty remarkable" for a custom home in Asheville, which is a pricey area, says Karon Korp, a writer who moved into the house in July.

Korp says she and her husband, Russ Martin wanted primarily an energy-efficient home. They're not particularly sensitive to chemicals, but they were drawn to Brenner because of his modern aesthetic and green building enthusiasm. She says they're thrilled their house is made of a renewable, toxic-free material and hope it sets an example for the nation.

"Hemp could replace tobacco if it were legalized," says Martin, Asheville's GOP mayor from 1993 to 1997. He says some area tobacco farms have gone bust.

Martin says they have spent less than $100 a month so far to cool the home, which has 3,000 square feet plus a garage. It has 12" thick walls, Energy Star appliances, dual-flush toilets, high-performance windows and LED lights. Korp says they might add a windmill, because the house sits atop a mountain.

They say they have fantastic views. "We seen the sun rise," he says. She adds, "and the sun set."