and fell out of favor after World War I, although it is seeing a resurgence today. Cob is one of the simplest and least expensive building techniques available, though it is typically very labor-intensive. Cob's other great advantage is versatility; It can easily be shaped into any form. While cob building was falling out of favor in England by the late 19th century, thousands of cob structures have endured to the present (20,000 in Devon, England alone). It is estimated that from one third to one half of the world's population lives in earthen dwellings today. Although typically associated with "low-rise" structures, in Yemen and other Middle-Eastern countries it has, for centuries, been used in "apartment" buildings of eight stories and more. Cob-like mixes are also used as plaster or filler in several methods of natural building, such as adobe, earth bags, timber frames, cordwood, and straw bales. Earth is thus a primary ingredient of natural building. Recently, a Cob Code was voted into the US International Residential Code. Cordwood Cordwood construction is a term used for a natural building method in which "cordwood" or short lengths of pieces of debarked tree are laid up crosswise with masonry or cob mixtures to build a wall. The cordwood, thus, becomes infill for the walls, usually between posts in a timber frame structure. Cordwood masonry can be combined with other methods (e.g., rammed earth, cob or light clay) to produce attractive combinations. Cordwood masonry construction provides a relatively high thermal mass, which helps to minimize fluctuations in temperature. Earthbag Earth is the most typical fill material used in bag-wall construction techniques. This building method utilizes stacked polypropylene or natural-fiber (burlap) bags filled with earth or other mixes without a stabilizer, other than clay, to form footings, foundations, walls and even vaulted or domed roofs. In recent years, building with earth bags has become one of the increasingly practiced techniques in natural building. It facilitates self-contained, often free-form rammed-earth structures. Its growing popularity relates to its use of an abundant and readily available often site-available material (earth) in a potentially inexpensive building technique that is flexible, and easy to learn and use. However, because earth is a poor insulator, in more extreme climates other filler variations are now being explored, substituting pumice, rice-hulls or another material with better insulating value for all or part of the earth (see also Rice-hull bagwall construction). Hempcrete