Alternative Material Cuts Energy Costs
Though rammed earth and other alternative construction techniques have been in use around the world for hundreds of years, they are not well known outside a small but growing community of architects, designers, contractors, and owner-builders who are exploring more ecologically friendly and energy-efficient methods of making living spaces.
Rammed Earth DefinedRammed-earth construction requires soil (ideally, a mixture of 70 percent sand and 30 percent clay), water, formwork, and tamping equipment for compressing it to 100 to 110 pounds per cubic foot. The massive walls created in rammed-earth buildings buffer the interior from the outdoor temperatures.
Such buildings are much slower to heat up than conventional ones, and, because of their great thermal mass, they dissipate heat to the interior very slowly. Typical insulation values are R-0.25 per inch or R-4.5 for a typical 18-inch wall, according to U.S. Department of Housing and Urban Development documents. The real efficiency lies not in the insulation value, but in the massive structure's ability to reduce indoor temperature swings.
"With a couple hundred tons of thermal mass," said Wuelpren, "the temperature inside a rammed-earth house may fluctuate only four or five degrees during the day [without mechanical heating or cooling], whereas the outdoor temperatures may vary 35 degrees in a 24-hour period here in Tucson.
"Compare that with lightweight frame construction, which could fluctuate 25 degrees in a day, without air conditioning - and with a lot more hot and cold spots. Rammed earth greatly reduces extremes in interior temperature variation."
Wuelpren said that even though energy savings are not as dramatic as they once were prior to higher SEER ratings and improved A/C systems, "you are still looking at a 30-percent savings in energy costs, when compared to a conventional wood frame stucco house."
What About The Ceiling?"Rammed earth is an excellent building material," said Carl Rald, energy program coordinator for the city of Tucson, "but if the ceiling and roof are leaking energy, it doesn't matter how good your wall is. If the ducts in a 150 degree attic are leaking, you can lose 25 percent of your airflow, regardless of the walls' thermal mass."
Rald, a former University of Arizona engineering professor who built the first commercial rammed-earth building in Tucson, said he "loves the material, but it's just one of many possibilities.
"People always ask me, â€˜What's the best material?' There is no such thing as a good or bad material, only a good or bad way of using it.
"You have to look at the whole house. You can have the best walls, the best ceilings, the best windows, but if the house doesn't work together as a system, it's like forgetting to put the plug in the bathtub before you put in the water. You have to look at every factor.
"What the walls are made of is not as important as how the roof and ceiling are constructed. You lose much more airflow through the roof than through the walls - up to 10 times more through
the ceiling than through the walls. That's because the sun is right above us, almost perpendicular to the roof, especially in this climate. But when the sun hits the walls, it's at a low angle."
Rald indicated that a metal roof is fine for a rammed-earth house, if it is properly insulated and ventilated. A dark-painted metal roof, however, would be a big mistake. "With a roof," said Rald, "whether on a rammed-earth house or a conventional frame and stucco, color is everything."
Sidebar: Rammed Earth From An HVAC Contractor's ViewpointFor more than 10 years, Jeff Hamstra, owner of Hamstra Heating and Cooling in Tucson, Ariz., has been designing and installing heating and air conditioning units for rammed-earth homes.
"We work with the thermal mass of the walls to meet the home's cooling needs," Hamstra said. "Typically, even on the hottest days, air conditioning use is delayed until the evening hours.
"The comment I often receive from homeowners is that their A/C doesn't run much, if at all, during the day, but runs more than they're accustomed to between 8 p.m. and midnight. Then it typically shuts off because the night air cools the house back down."
Hamstra's firm did $4.7 million of business last year, "mainly residential with some light commercial." Rammed-earth construction is as yet a small fraction of total sales. He said there are no design differences between working with a conventional framed stucco house and a rammed-earth structure. "R-values, however, are very difficult to determine. No one has actually set standards for calculating them for rammed earth - not that I know of."
Hamstra calculates manual J load, just as he would with a conventional building. "I look at the heat loss and gain through doors, windows, the roof to determine the unit size, how much air to put into each room.
"What's different about rammed earth is that temperature change happens very gradually. In the summer, it heats up outside, but only very, very slowly inside."
If the house is designed properly, with overhangs that shield the interior from summer sun and windows that let sun enter the home during the winter, the cooling and heating effects of rammed earth are enhanced.
"In the winter," said Hamstra, "mornings are like those in a conventional wood frame stucco house - it's cooled down over the night and you need to put the furnace on to take the chill off. During the day, the house heats up with the sun. With rammed-earth houses, it's not just the exterior walls that create the thermal mass, but there are a lot of internal masses like fireplaces and niches and thick, interior walls that are exposed to the sun and help keep the heat from drifting. They then dissipate the heat very slowly in the evening, adding to the overall comfort."
Even though Tucson is known for its scorching summers and not for its chilly winters, Hamstra said, "On those days when it is cold, you'd be challenged without a heat source," if you were simply counting on rammed-earth walls to store heat and windows to let heat in during the day.
"Besides, if you have an A/C system for cooling, you can easily pump heat through the system for little or no additional cost. But if you're relying on an evaporative cooler, you'd need something more."
Sidebar: Rammed-Earth ResourcesTo learn more about rammed-earth construction, visit the following Web sites:
This site lists 27 links to rammed-earth, adobe, cob, and earth-sheltered educators, builders, equipment vendors, and resource centers.
This site contains information on compressed earth block technology.
The site of author-consultant-innovator David Easton's Napa, Calif., firm, contains a beautiful photo gallery of completed and ongoing award-winning rammed-earth homes and wineries.
The site contains a concise overview of rammed-earth construction.
Publication date: 11/08/2004