Rammed earth buildings are made from bricks of compressed soil, often mixed with a percentage of cement to help prevent weathering and water damage. Modern rammed earth construction often involves digging out part of a hillside, converting the resulting subsoil into rammed earth bricks, and then using them to build a home that at least partially fills the hole dug into the hill. That way, the bulk of the buildng material does not have to be shipped in, and the buried part of the house is extremely well insulated. When combined with passive solar energy systems, this kind of construction can result in very low heating bills – and in some cases none at all, even in winter.
Rammed earth construction is an ancient building method, dating back to 10,000 years ago in western asia. The Romans used this very practical technique throughout their empire, and it was only when mass-produced brick and lumber became available that it fell out of favour.
During the 1920s through the 1940s, millions of dollars were spent by the US Government and several western universities researching rammed earth construction. In 1936 on a homestead near Gardendale, Alabama, the United States Department of Agriculture constructed an experimental community of rammed earth buildings with architect Thomas Hibben. The houses were built at a very reasonable cost and sold to the public, along with tracts of land sufficient enough for a garden and small livestock plots.
Interest in rammed earth fell after World War II when the costs of modern building materials dropped. Rammed earth became viewed as substandard and it suffered from the prejudice that using earth technique seemed too basic in the face of new technology and too dependent on labor intensive methods. However, rammed earth is not only an economically viable construction technique, it results in pleasant, energy-efficient buildings.
Rammed earth constructs typically feel cool, quiet and solid. These are virtues of the building material itself: the density and thickness of rammed earth means it has a very low rate of thermal conductivity. Warmth takes almost 12 hours to work its way through a 360mm (14-inch) thick wall, even though they are not good insulators (unprotected exteriors are usually made as cavity walls, or sprayed with an insulating render) . The walls provide good thermal mass, which helps keep indoor temperatures stable, particularly in regions with dramatic daily temperature changes.
Typically rammed earth walls are about 360 mm (14 inches) thick making them ideal for humidity control and excellent noise barriers from traffic, furnaces, compressors, fans or ducts. Rammed earth also allows more air exchange than concrete structures, as the material mass allows the building to ‘breathe’, avoiding condensation issues without significant heat loss. The material is reusable, biodegradable and highly fire resistant. Properly-built rammed earth can withstand loads for thousands of years as the history of rammed earth structures around the world has proven (sections of the Great Wall of China are rammed earth).
Compressing the earth can be done manually using a tamper made of a heavy, flat-bottomed plate connected to a long vertical handle. Using a pneumatically-powered tamper, the material can be compressed with much less manual labour. Although the cost of material is low, constructing rammed earth without mechanical tools can be a very time consuming project; however with a mechanical tamper and prefabricated forms it can take as little as two to three days to construct the walls for a 2000–2200 sq ft house. Because rammed earth structures utilize locally available materials, they typically have a low embodied energy rating and generate very little waste. Earth used for building is a widely-available resource and harvesting it for use in construction has minimal environmental impact. The soils used are typically subsoils, retaining organic topsoil for agricultural use.
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