Free-standing walls enclose space to create privacy, make an area secure, or improve the microclimate. For minimal environmental impact, build an earth wall. Earth (aka soil) can be shaped into adobe blocks that are then stacked to form a wall, tamped between temporary wooden forms to make rammed earth, or simply plopped into a free-form wall of cob (fancy language for blobs of mud and straw). For a higher-tech and more formal-looking wall, look into new soil-cement blocks that are coming onto the market. They're made from natural materials, have a high recycled content, and require much less energy to manufacture than the conventional concrete blocks they replace. And don't forget the usefulness of living hedges and vines in creating privacy.
Of course, you can use more traditional materials such as wood, metal, or masonry (brick, stone, concrete block, or plaster) to build a wall. Masonry walls are durable and handsome, but they're also more expensive and often higher in environmental impacts than the other types. A traditional masonry wall, made almost entirely of cement-like materials has all the drawbacks inherent to that material: strip mining, carbon dioxide production, and embodied energy. If you decide to build a wall with one of these materials, build it so it will last for decades — maybe even centuries. Spread the inputs over the lifespan, and you'll have a very sustainable structure. Using high-volume fly ash concrete for the footings will help as well.
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