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"We originally proposed using concrete for the walls because we like the plain style and directness," the owners say. Instead of concrete, Smith suggested rammed earth, a material that combines some of the physical properties of concrete, but is less "psychologically cold." The 16-inch-thick walls' thermal mass helps to insulate the interior from heat in the summer and cold in the winter.

"Rammed earth is complicated," says Smith, "so have the right expectations, do the research, and hire the right people." David Easton—who Smith calls "The Godfather of Rammed Earth"—headed the contracting team that built the walls.