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Cob, cobb or clom (in Wales) is a natural building material made from sand, clay, water, some kind of fibrous or organic material (straw) and earth. Cob is fireproof, resistant to seismic activity, and inexpensive. It can be used to create artistic, sculptural forms and has been revived in recent years by the natural building and sustainability movements.
In technical building and engineering documents such as the Uniform Building Code, cob may be referred to as an "unburned clay masonry" when used in a structural context. It might also be referred to as an "aggregate" in non-structural contexts, such as a "clay and sand aggregate" or more simply an "organic aggregate," such as where the cob is an insulating filler between pole and beam construction.
Cob is an ancient building material, that may have been used for building since prehistoric times. Some of the oldest man-made structures in Afghanistan are composed of rammed earth and cob. Cobwork (tabya) was used in the Maghreb and al-Andalus in the 11th and 12th centuries and was described in detail by Ibn Khaldun in the 14th century.
Cob structures can be found in a variety of climates across the globe. European examples include:
Traditionally, English cob was made by mixing the clay-based subsoil with sand, straw and water using oxen to trample it. The earthen mixture was then ladled onto a stone foundation in courses and trodden onto the wall by workers in a process known as cobbing. The construction would progress according to the time required for the prior course to dry. After drying, the walls would be trimmed and the next course built, with lintels for later openings such as doors and windows being placed as the wall takes shape.
The walls of a cob house were generally about 24 inches thick, and windows were correspondingly deep-set, giving the homes a characteristic internal appearance. The thick walls provided excellent thermal mass which was easy to keep warm in winter and cool in summer. Walls with a high thermal mass value act as a thermal buffer inside the home. The material has a long life-span even in rainy climates, provided a tall foundation and large roof overhang are present.
When Kevin McCabe built a two-story, four bedroom cob house in England in 1994, it was reputedly the first cob residence built in the country in 70 years. His methods remained very traditional; the only innovations he added were using a tractor to mix the cob itself, and adding sand or shillet (a gravel of crushed shale) to reduce the shrinkage.
From 2002 to 2004, sustainability enthusiast Rob Hopkins initiated the building of a cob house for his family, the first new one in Ireland in about one hundred years. It was undertaken as a community project, but destroyed by an unknown arsonist shortly before completion. This house, located at The Hollies Centre for Practical Sustainability in County Cork, is being rebuilt (2010). There are also a number of other modern cob houses completed and more planned including a public education centre.
In 2000–2001, a modern, four-bedroom cob house in Worcestershire, England, designed by Associated Architects was sold for £999,000. Cobtun House was built in 2001 and won the Royal Institute of British Architects' Sustainable Building of the Year award in 2005. The total construction cost was £300,000, but the metre-thick cob outer wall cost only £20,000.
In the Pacific Northwest of the United States there has been a resurgence of cob building both as an alternative building practice and one desired for its form, function and cost effectiveness. There are more than ten cob houses in the Southern Gulf Islands of British Columbia built by Pat Hennebery, Tracy Calvert, Elke Cole and the Cobworks workshops.
In 2007, Ann and Gord Baird began building a two-story cob house in Victoria, British Columbia for an estimated $210,000 CDN. The 2,150 sq. ft. home includes heated floors, solar panels and a southern exposure for passive solar heating.
In 2010, Sota Construction Services in Pittsburgh, Pennsylvania, completed construction on its new 7,500 sq. ft. corporate headquarters, which features exterior cob walls, along with other energy-saving features like radiant heat flooring, roof-mounted solar panel array, and daylighting features. The cob walls, in conjunction with the other sustainable features, helped the building earn a LEED Platinum rating in 2012, and it also received one of the highest scores by percentage of total points earned in any LEED category.
The building process known as "Oregon Cob" is one which was refined by Welsh architect Ianto Evans and researcher Linda Smiley in the 1980s. Oregon Cob integrates the variation of wall layup technique which uses loaves of mud mixed with sand and straw with a rounded architectural stylism. They experimented with a mixture of cob and straw bale construction, called balecob.
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