Dirt! The Movie can be acquired for community screenings. See www.dirtthemovie.org for more information.

The dirt floor is making a comeback and so is the mud wall. For millennia, earth in its many forms was a favored building material, and though mud wall construction may sound like an anachronism, it’s actually a remarkably durable, energy-efficient technology. Adobe, rammed earth and cob are ancient, time-tested materials that have become increasingly popular among the eco-conscious. But the ubiquity of wood frame housing, coupled with the difficulty of obtaining proper permits for less conventional structures, puts earthen homes out of reach for most Americans.

Lately, though, a new generation of green builders is pioneering alternative ways to make the phrase “living close to the Earth” more than just a figure of speech. One example can be seen in Dirt! The Movie, a new documentary that rehabilitates the image of the magical substance we’ve grown apart from in our antiseptic cities and houses. In a particularly intriguing sequence, a young couple is shown driving up in a small electric utility vehicle to a weathered ranch house in the Bay Area. Once inside, he begins hand-mixing thick, sticky, grayish-brown mud in a big red bucket, which she carefully spreads on the floor with a trowel. Linseed oil, imbued with a deep maroon pigment, is spread on next for extra color and shine. Lastly, it’s sealed with a coat of wax to make it waterproof and durable.

The couple, Kevin Rowell and Marisha Farnsworth, are laying the dirt floor in the bedroom of their cinder-block garage home. They founded The Natural Builders in 2005 and perfected the art of mud plastering for eco-friendly clientele. The floor they install in Dirt! is made of earth excavated from beneath the topsoil, amended for a proper ratio of sand and clay and fortified with horse manure. That last ingredient may dissuade the squeamish (and can be substituted if the client insists), but there are few better binding agents.

“We use the wonderful machine, the horse, to process our straw,” Rowell says in the film. “It’s an incredibly fine fiber. And in the process it’s adding enzymes and other proteins to the manure, which act as a natural glue. So as this dries, it dries with an incredible hardness.” Compared to concrete, however, the end result is much softer and more forgiving to walk on. It’s also a better insulator, making it feel warmer to your feet.

Though still a fringe practice in this country, dirt floors and mud wallsor “earthen floors” and “natural finishes,” in industry parlanceare growing in popularity. Aside from adding an earthy, non-uniform beauty to living spaces, the main benefits accrue to the local and global environment: no toxic chemicals are brought into the home, the floor can function as a passive thermal mass when exposed to sunlight, and most of the materials can be dug up right outside for a minimal ecological footprint. And “they’re dirt cheapliterally,” says Farnsworth. Floors run about a dollar a square foot ($5 including labor).

The green building movement is strongest in the Western U.S., but is gaining footholds in the East. Rowell co-directs Kleiwerks International, a nonprofit that advocates natural building, with Asheville resident Janell Kapoor. Kleiwerks has constructed eco-friendly buildings and installed earthen floors and walls in western North Carolina and Virginia. They also provide workshops and training sessions in Asheville for aspiring natural builders.

At the time of this writing, Rowell was in Haiti, representing Kleiwerks and Builders Without Borders. “We trying to see if there’s a way to involve some of the techniques of ecological construction in the rebuilding process,” he said by phone.

Whether in Haiti or the U.S., the use of local materials for construction can have a tremendous positive impact on the environment, especially if they replace concrete, which is a notoriously carbon-intensive material. There are also definite rewards for homeowners willing to undertake their own earthen installations, according to Rowell in Dirt! “You get the dirt high,” he says. “It’s a really different experience than VOCs [volatile organic compounds] and petrochemicals. And I’d like to think that it lasts longer. Everybody could get it if they got their hands dirty.”

Correction (April 19, 2010): The correct URL for Dirt! The Movie is www.dirtthemovie.org.