Earthships are a form of construction that stretches the definition of natural building while maintaining true to its spirit. Developed by New Mexico architect Michael Reynolds in the late seventies and early eighties, earthship construction makes good use of a plentiful local materialold tiresin an integrated design that also incorporates passive solar, earth-berming, photovoltaics, and water-conservation. It is a form of autonomous or off the grid housing, and an expression of one of the most radical and holistic philosophies of ecological living (Chiras, 2000). Most of the public attention given to earthships focuses on their walls made of engineered rubber-encased adobe building blocksthat is, old car and truck tires pounded full of dirt from the excavation (Ehrhardt, 2002). The walls of stuffed stacked tires are plastered or stuccoed, creating an incredibly strong, earthquake resistant, fireand termite-proof structure. The tire walls, oriented to the sun and earth-bermed on the north side, provide the great thermal mass that is the basis of its energy-efficiency. The earthship is, first and foremost, a thermal mass building, and secondarily, a passive solar building. Earthships are touted as providing comfort in any climate by designing a specific balance of mass and insolation that is appropriate to the climate in question. The Potter earthship near Bancroft Ontario uses only a small wood stove as supplementary heat periodically during the winter months (Berczi, Minke, & Sheen, 1996). The tires, encased in earth and plaster, do not emit harmful substances into either the air or the soil and water table, and are a building of choice for many chemically-sensitive people.