A recent cradle-to-grave study from the Portland Cement Association (PCA) compares the long-term environmental impact of lightweight concrete houses with conventional, 2x4 wood-framed houses, and finds a close race. Using guidelines set forth by the International Organization for Standardizations, researchers with the PCA's Construction Technologies Laboratory in Skokie, Ill., modeled two 2,450-square-foot homes--the first built using 2x4 frame construction, the other using lightweight concrete masonry units, made with expanded shale, clay, and slate. The researchers note that their analysis of life cycle inventory for the homes is partial, because it does not include the embodied energy (the amount of energy required in the production of a material) from materials other than concrete that were used in the home. What it does include is energy use, material use, and emissions to air and solid waste generation over the 100-year life cycle. Their results found that 99 percent of all life-cycle energy usage over that time span is owner occupant energy use. They note that although the lightweight concrete house requires more initial embodied energy for production of cement and concrete, the lightweight concrete house boasts a lower occupant energy usage once complete. In cold climates, such as Detroit, this means that after 11 years, the cumulative energy use of the wood-frame house exceeds that of the concrete house--and stays that way over the rest of the 100-year model period.