Approximately half of the world's population lives in earthen structures, but the construction method is frequently overlooked by builders within the United States—except in the dry, warm climate of the desert Southwest. Earthen walls provide thermal mass qualities that regulate temperature and humidity passively, which keeps the indoor environment cool in the summer and warm in the winter.
Compressed earth blocks (CEBs) are an eco-friendly construction material, similar in composition to adobe bricks but without the need for sun-drying, that is becoming more common in semi-arid climates such as Colorado's. Made from a mix of sand, minimally expansive clay, and a small amount of cement that makes them more resistant to moisture and therefore more stable than adobe, CEBs offer a low-cost, mold- and insect-resistant building alternative that can be made using locally available resources.
But would CEBs work in Montana, where the winter weather is even more extreme?
A team of CU researchers from the Mortenson Center in Engineering for Developing Communities was asked to answer that question for the Department of Energy and Mineral Development in the Bureau of Indian Affairs (BIA) and, in the process, has helped create a sustainable housing solution for the Crow Nation in southeastern Montana.
The pilot project caught the attention of President Obama as the BIA's best example of a successful stimulus project last year. The project also has received funds from U.S. Housing and Urban Development and the BIA's Housing Improvement Program since the Crow Nation has a shortage of about 1,900 homes and an unemployment rate of more than 40 percent.
Crow Chairman Cedric Black Eagle and project manager Larry Lee Falls Down traveled to Washington, D.C., this winter to meet with President Obama, who has shown a keen interest in the Crow and was even adopted into the tribe during his 2008 election bid.
Four homes have been completed using the compressed earth blocks as of March, and as many as 12 more are expected to be completed in the next year.
Using two "wythes," or a double-row of blocks with insulating material in between to comprise each wall, along with passive solar design elements and a ground-source heat pump, the homes' heating costs are expected to be only about one-tenth as much as those associated with frame construction, according to Tom Bowen, director of sustainable housing projects for the Mortenson Center at CU-Boulder.
"We're seeing strong positive evidence that this is the most likely to succeed approach to alleviating the tribe's housing shortage because it allows the tribal members to learn a new technical skill that is based on locally available materials and it will save the families who buy these homes significant money on energy costs," says Bowen, who has been consulting with the Crow for the past two and a half years to get to this point.
Bowen has led a team of students in the civil engineering department in analyzing the quality of the sand and clay available in the area and determining the right mix of materials and pressure to make the CEBs structurally sound. The final recipe includes four parts clay from the Big Horn Mountains, one part sand from Crow Agency, 5 percent Portland cement to make the blocks moisture resistant, and 8 percent water.
The CU researchers expect to continue working on the project for several more years with their focus being on teaching Crow project leaders the steps to produce high-quality blocks and helping to develop their enterprise so that it can be self-sustaining.
Bowen says the Mortenson Center team also hopes to be able to introduce the new materials into commercial development on the Crow Reservation, and to expand into similar projects with other Indian nations.