Rammed earth construction refers to using actual earth as a building material. It often involves a particular mixture of soil and clay, with other added materials, that is compacted and allowed to cure in the shape of walls. Though much more energy efficient that a wood-framed home, one made of rammed earth will likely cost more at the outset, especially in Western nations, because of the labor involved. This type of construction is best suited to dry climates like the desert Southwest of the United States, although it can be used in other, rainier climates as well.
The use of rammed earth as a construction material dates back to ancient civilizations. It was first used in desert areas where trees and therefore lumber were in short supply, and trade was limited. Despite its age-old origins, rammed earth has been gaining in popularity in recent years, as people seek to use materials from sustainable sources, and ones that have a minimal negative impact on local environments. Not only is earth an abundant and sustainable resource, but it has the potential to significantly reduce energy consumption in buildings that are constructed from it.
The main disadvantage of this type of construction is that it is highly labor-intensive. Most often, forming walls out of rammed earth involves filling wooden forms with soil, packing it down to about half its original height, adding more soil, packing it down, etc. This is done until the desired wall height has been reached. The exact materials used are not simply topsoil from the building site either, but must contain the correct proportions of clay, moisture, gravel, and sand in order to achieve optimum results.
This mixture often has a stabilizer added to it, which can be any of several materials, including ordinary concrete. Some prefer to avoid the use of concrete, however, since it production involves the emission of pollutants into the air. When the mixture has been compacted sufficiently, the wooden forms are removed, and the the material is allowed to dry in the sun for a period of days. Exposed outer walls are often sealed to prevent damage from water seeping through the material, since it is so porous.
One of the best features of finished rammed earth construction is a property known as its thermal mass. Put simply, rammed earth is slow to heat up during the day, but retains its heat for a while at night. This makes it an ideal construction material for desert climates. It is also very long-lasting, as demonstrated by rammed earth buildings that are hundreds or even thousands of years old, but are still standing.