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Case Study: The Nauhaus

By Tracy Rose on 03/22/2010

If you drive by the Nauhaus home under construction in a quiet neighborhood off Brevard Road in West Asheville, you might not notice anything radically different about it.

Hemp team: Mix bags of hemp fiber with a lime-based binder, add a crew of enviro-builders, and you get a hempcrete house in Asheville.
photos by Jonathan Welch

But if you could peer into the walls, even the untrained eye probably could spot something unusual. On a frigid December day, a 16-inch-thick gray wall that looks like it's made of ground-up straw mashed together into blocks is taking shape on the house's ground floor. But instead of straw, the blocks actually are made of hempcrete — chopped hemp fibers called shiv that have been mixed with a lime-based binder onsite and poured into forms.

The hempcrete walls offer perhaps the biggest wow factor about the Nauhaus, a prototype home on Talmadge Street being constructed by the Nauhaus Group, a collection of local companies offering design, building and development services from its West Asheville headquarters.

What may be more remarkable, however, is the project's goal — carbon neutrality — and the combination of high-tech building science and natural materials being used to achieve it.

"Our agenda is basically affordable, carbon-neutral building solutions," explains Clarke Snell, the managing director of The Nauhaus Institute, the nonprofit research/education arm of the group.

Snell has deep roots in natural building, an approach that uses close-to-the-earth materials such as bales of straw and, well, earth. He's authored two books on the subject (The Good House Book: A Common-Sense Guide to Alternative Homebuilding and Building Green: A Complete How-To Guide to Alternative Building Methods), the latter with Nauhaus architectural designer Tim Callahan.

But while Snell loves natural materials — and is employing them when practical in the Nauhaus — he says that getting serious about carbon neutrality and solving environmental problems means taking a more technical approach than natural building alone affords. The labor-intensive nature of natural building usually relegates the techniques to owner-builders, he says, since it's either very expensive or time-consuming otherwise. The Nauhaus Group aims to combine the advantages of both the natural and high-performance building worlds into a hybrid system.

So hempcrete — chosen largely for its vapor-permeable quality — has a foot in both worlds: It's a natural material, but one that's been lab-tested and is acceptable to building-code officials. Essentially, hempcrete walls can absorb or shed water in response to the changing humidity levels in the air. For its part, the lime wicks water away from the cellulose and inhibits mold growth, Snell notes. On the downside, since hemp can't legally be grown in the United States, the product whose brand name is Tradical Hemcrete has to be imported from England.

"This is the only product that we know of in the world right now that has the properties that we want that you can buy in a bag," says Snell. "And that's the transition that we're trying to make … out of these sort of impractical but wonderful site-made materials into comparable commercial products that can be installed quickly for a quoted price."

Strong stuff: Hempcrete walls have good insulating properties and are vapor permeable — the material can absorb or shed water in response to changing humidity levels in the home.

But why aim to be carbon-neutral in the first place? Snell says that once you accept the science of climate change, the solution is simple: Reduce the amount of fuel you're burning to cut carbon-dioxide emissions. And since buildings make up a significant part of our carbon footprint, builders have it in their power to address a large part of the problem.

To achieve maximum energy efficiency, the Nauhaus group turned to a German certification program called Passive House. Buildings constructed to this rigorous standard are about 80 percent more efficient than those built to code, Snell says.

"A Passive House is a very well-insulated, virtually air-tight building that is primarily heated by passive solar gain and by internal gains from people, electrical equipment, etc.," notes the Web site for the Passive House Institute U.S., an Illinois firm that certifies Passive Houses in this country.

To say the Nauhaus meets the standard seems like an understatement.

"The basic idea is really simple," Snell explains, pointing to the insulated concrete slab on which the house rests. "You wrap the inside of the house in lots of insulation."

That starts with insulating the slab itself, a detail Snell says often goes overlooked by folks who are trying to be energy efficient. In the Nauhaus, polyurethane foam — four and a half inches of it, sprayed on a sheet of plastic under the slab — brings its R-value to 30 (compared to an R-value of zero to 5 to meet code requirements).

On the outside of the slab, the Nauhaus uses insulated, autoclaved aerated concrete blocks for part of the stem wall. The outside wall is sprayed with more insulation.

Inside, the insulation extravaganza continues with the 16-inch-thick hempcrete, which provides additional insulation around the wood framing and brings the R-value to about 40 — significantly higher than the code requirement of R-13. High-performance windows and doors from Serious Materials minimize heat loss. The roof will be insulated to R-75 (almost double the code requirement of R-38) with a combination of cellulose and high-performance structural insulated panels from a local company, Eco-Panels. In addition, breaks in the insulation layer — called thermal bridges — have been eliminated throughout the house.

With the low-arcing December sun shining through the framing in the unfinished upstairs space, it's easy to imagine how toasty the house will be in winter once another design feature — passive solar — comes into play.

In another nod to natural materials, the Nauhaus will feature compressed earth blocks made on site with soil dug up for the foundation that's then mixed with a small amount of Portland cement and shaped with a hydraulic press, Snell says. The blocks will be used for many of the interior partition walls and for much of the finish floor to provide thermal mass, which is needed in passive solar design to hold and then release heat.

If all the Passive House strategies are employed effectively, then a building doesn't need the normal heating and cooling systems, notes Callahan.

For additional heating, the all-electric house will use a mini-split heat pump, powered by rooftop solar electric panels, which will also power the heat-pump water heater. Since the house will be so airtight, an energy recovery ventilator will be used to bring in and circulate fresh air. The ERV and mini-splits will work together to dehumidify the house in the summer.

The building, which includes an apartment — and therefore two kitchens and heating/cooling systems — needs only a 2-kilowatt solar-electric system to produce, on site, 100 percent of its electricity needs, Snell reports.

Though Callahan reports that the house will cost 5 to 15 percent more to build than a regular house, the energy costs are projected to be so much lower that a buyer could put the money saved on monthly energy bills into the mortgage and still wind up with a lower overall monthly housing cost.

Natural materials will show up in other parts of the house as well. A lime-based plaster will go on the exterior hempcrete, while earth-based plasters will be used on the interior, along with Asheville-based Earthpaint products. Horse-logged hemlock (cut because it was infested with woolly adelgid) from Mountain Works Sustainable Development based in Boone will be used for exterior fascia of the roof overhangs, while all interior wood will either be horse-logged or reclaimed.

Although they weren't able to incorporate local framing wood, Snell notes that the wall system uses less wood due to wider spacing (24 inches versus the typical 16 inches) and less bracing, thanks to the structural support of the hempcrete.

Another green feature of the house is its small size — about 1,400 interior square feet, which takes less energy to heat and cool. (The extra-thick walls bring the home's footprint to about 1,700 square feet.)

An integral part of the home's design is the "urban homestead" landscaping planned for the one-eighth-acre lot. Plans reveal a rain-catchment cistern, pond, butterfly garden, raspberry trellis, grape arbor, blueberry bushes and shitake mushroom logs.

"The 'green' in 'green building' is plants, yet landscaping continues to be an afterthought in most green projects. For us, plants are integral to the home: They're there to nurture the inhabitants in exchange for the inhabitants nurturing them," Snell offers.

In another experiment, the Nauhaus team plans to try out insulation from mycelium, the "roots" of mushrooms, in the walls and roof of a garden shed on the property.

Once the Nauhaus prototype is finished, the group's systems engineer Jeff Buscher and his family will move in to continue gathering information about the house's performance.

The knowledge the Nauhaus Group gains from the prototype house will be put toward building future homes, mixed-use buildings, apartments and larger commercial projects.

The project has excited plenty of local interest.

"I think one of the unique aspects of this project is the number of people who have been involved in making it happen," says Callahan. From the house's conception to the design and materials, he notes "It's been a real community effort."

One of the project's volunteers, massage therapist Renee Kirzner, helps promote the Nauhaus via social media like Twitter. "It's empowering to know that there are models like the Nauhaus to address climate change so completely right here in our neighborhood," Kirzner says. "I definitely think their approach is so hopeful and inspiring."

Learn more about the Nauhaus at http://www.thenauhaus.com. You can also check out the Facebook page (search for The Nauhaus Institute) and get Twitter updates (@theNauHaus).

[Tracy Rose is an Asheville-based freelance writer and editor.]