Trending Now: Rammed Earth Makes Resurgence

August 18, 2015 • Scott Ramser, AIA

What do the Great Wall of China, the Alhambra in Spain and Earthships in New Mexico have in common? It’s called rammed earth.

It’s a construction method that, in a most basic sense, uses hard-packed dirt to create both interior elements and exterior walls of buildings. Today’s approach uses a cross-grade of soils like sand, gravel and clay mixed with a stabilizer like cement or asphalt to help the soils stick together.

This soil mixture cannot be allowed to dry, so it is immediately poured into a form, typically made of plywood, to a depth of 4-10 inches. The earthen mix can be rammed by hand, but professional builders use machines that are attached to air compressors. When a section is under the right amount of pressure, the dull tamping sound is replaced by a ringing one. A finished cement-stabilized rammed earth wall is about as strong and durable as concrete. Additional stability can be achieved by embedding wood, bamboo or steel framing within the wall.

A Brief History

Rammed earth dates all the way back to 5000 BC, used mostly by the ancient Chinese. But it has shown up even in the United States’ recent history. In 1926 the U.S. Department of Agriculture issued official instructions for building rammed earth homes, which were used largely by farmers during the Depression years. As the U.S. economy recovered, interest faded and then died out almost completely after World War II when modern building material prices dropped.

But a method of construction that stands for over 2,000 years (in the case of the Great Wall) is bound to come back around. And it is. From luxury homes to our own zoo entrance and administration building at Phoenix Zoo, the rammed earth method is again gaining in popularity.

 

David Schacher Photography LLC

 

Benefits

Obvious benefits of rammed earth include the character and eccentricity it adds to a home or building. The final product can evoke the harsh beauty of rugged countryside or make a strong artistic statement in an urban setting.

But there are also more practical benefits. By far the most popular is thermal mass. The walls work basically like a battery, storing thermal energy as the temperature rises and keeping it from being transferred into the interior of the building during the heat of the day. This keeps the interior nice and cool. Then as night falls and the outside temperature drops, the rammed earth wall slowly radiates that stored heat into the building’s interior, preventing it from becoming chilly. This effectively empties the thermal battery and allows it to recharge the next day, in a continuous cycle.

This modulating effect can keep interior temperatures comfortable year-round, especially in a hot, arid climate, where there is a significant difference between the daytime highs and the nighttime lows.

The method is cost-effective, and the use of regional materials minimizes transportation costs of materials and people. Usually only one experienced professional is necessary on-site, and the rest of the labor can be done by an untrained crew or even community members, further reducing costs.

It’s also a more sustainable option, generating little to no waste. And the wooden forms can be reused, decreasing demand for lumber. Research and experimentation with additive support materials could also increase sustainability as builders opt for these over cement.

Other benefits include fire resistance, rodent and insect resistance and lower maintenance costs. The walls don’t even have to be painted!

So What?

If rammed earth is long-standing and more sustainable, why isn’t everyone using it? Well, even though it’s an older-than-the-hills method, there’s a significant lack of knowledge compared to modern building materials.

Additionally, there’s little to no regulation, which not only makes it challenging for engineers and architects during the design process, but also on bankers and insurers. This in turn can make it difficult for owners to get a mortgage or insurance, which can discourage the use of rammed earth.

It’s also worth mentioning that rammed earth works better as a building material if it can be kept dry, just like other masonry. This is usually achieved through large overhangs (which often times provide another opportunity for artistic design) and appropriate guttering.

But these challenges might not last long. Increased research and the push for more sustainable building options are making rammed earth construction the newest trend. Or at least as new as a method thousands of years old can be.

Sources:

History of Rammed Earth

How Rammed Earth Works

“Cheap, tough and green: why aren’t more buildings made of rammed earth?” by Daniela Ciancio, University of Western Australia

“Rammed-Earth Luxury Homes” by Amy Gamerman, WSJ

“All About Thermal Mass” by Martin Holladay, GBA