When Dirt Is a Good Thing

Subliette County Library (SCL) in Pinedale, WY, holds the distinction of being the first modern U.S. public building to use rammed earth as a construction material, according to Matt Thackay of Carney Architects, Jackson, WY. The 10,500 square foot main library is currently being expanded by 8000 square feet—with a mixture of sand, gravel, and clay poured into molds, which then will harden into the new wing's interior and exterior walls. Principal architect John Carney recommended the unique mix of materials to SCL director Daphne Platts when she told him that she wanted to make the building as green as possible.

The rammed earth method chosen by Carney is called SIREWALL® (stabilized insulated rammed earth), formulated by Meror Krayenhoff of Terra Firma Builders, Salt Spring Island, BC (www.sirewall.com). Rebar and foam insulation sits inside each custom mold, surrounded by 14–20 inches of earth. This nestles in rubble trenches with a four-inch bed of drain rock. The construction produces no waste.

This combination is especially suitable in harsh climates, explains Thackay. “Its large thermal mass helps mitigate those extremes,” he notes. The rammed earth walls hold the heat generated during the day and release the stored warmth at night. It also provides more protection against seismic activity, an issue for Jackson, which is on a fault line, and Pinedale has “some seismic activity.” Fixter Construction in Jackson is the general contractor.

Platts is quite pleased with the addition's aesthetic potential. The locally procured dirt can be dyed any color. SCL used iron oxide to produce a muted palette that reminds Platts of a sunset; plus, there aren't any fumes, she says. Molds can be formed into a variety of shapes, and the rammed earth counters in an area that will be rented out for private parties are particularly attractive, says Platts. Overall, the finished product will be contemporary but will marry quite well with the original log structure, says Thackay.

Platts says the process is expensive enough that SCL can't afford to make every surface SIRE-ed, so some portions are being traditionally built. Maintaining the log exterior on the original structure, however, costs SCL $12,000 every three years, adds Platts. Rammed earth requires no upkeep, but Platts admits she might not save money over conventional construction methods. “It's a very labor-intensive process.” It's an aesthetic choice—but a green one. The structure will include other moneysaving features like passive solar power generated by an array of photovoltaic collectors. It's also durable. When fully cured, the rammed earth walls are as strong as rock yet able to accept nails and be easily patched.

Terra Firma has a SIREWALL® training program in place for contractors and is a registered education provider with the American Institute of Architects. It even teaches participants how to choose the right earth.