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Hemp For Houses
by Rolf B. Priesnitz

Houses built from hemp have been found to use less energy, create less waste and take less fuel to heat than conventionally constructed homes.
Hemp as a building material.
Details of a naturally finished interior hemp plaster wall in an Irish cottage by hemp builder Steve Allin. Photo by Ivan Watkins

Hemp is perhaps best known for its Omega-3 and -6 fatty acids that make it a great addition to a healthy diet, and as a cotton substitute in ecologically-sound clothing and bedding. But it is also a versatile, environmentally-sound building material. 

A hemp crop can be grown without the use of herbicides or insecticides and produces up to four tonnes of material per acre per year. Hemp is categorized as a bast fiber crop. It has a stem consisting of an outer skin containing long, strong fibers and a hollow wood-like core or pith. Processing the stems results in two materials: hurds and fibers, both of which have properties that make them extremely useful in building construction. 

A variety of wood-like products, such as fiberboard, roofing tiles, wallboard, paneling, insulation and bricks, can be made from the compressed hurds. The fibers can also be used like straw in bale wall construction or with mud in a sort of modified cob style of building. 

Foundations can be made out of hemp hurds. A hemp plywood frame is filled with a hemp hurds combined with lime, sand, plaster, some cement and enough water to dampen, and then let to set for a day and to harden for a week. A sixth century hemp-reinforced bridge in France is testimony to the stone-like strength and durability of this material, which has come to be known as “hempcrete”. Hemp building boosters claim that hempcrete foundation walls are up to seven times stronger than those made of concrete, half as light and three times as elastic. This superior strength and flexibility means that hemp foundations are resistant to stress-induced cracking and breaking, even in earthquake-prone areas. The building material also is self-insulating; resistant to rotting, rodents and insects; and fire proof, waterproof and weather resistant.

Irish builder Henry O’D Thompson of The OldBuilders Company is a fan of using hemp and lime on old stone walls for insulation, condensation, sound muting and breathability. A restoration and conservation specialist who once lived in Canada, he says that lining walls with the hemp/lime mixture makes for a healthy house that doesn’t grow toxic mold.

Pipes can be made out of hempcrete and they, too have greater flexibility and greater elasticity than other those made from conventional materials, and they are resistant to cracking.

   

Stones can also be made out of hemp by wetting the stalk’s cellulose, and forming it into a hard black rock, which can be cut, drilled, cast, carved or formed into any shape.

When hemp hurds are mixed with a combination of lime products, they can produce a light weight insulating plaster, which can be cast around a timber frame or sprayed against a wooden or even stone form. Interior walls can be left exposed or finished with a natural paint. In France, the use of hemp plaster is common, partly because of its high insulation properties but also because it works in old stone buildings.

Steve Allin, a pioneer in the use of hemp as a building material in Ireland and author of the book Building With Hemp, mixes his own hemp products, which he calls Hemphab, and describes hemp plaster for interior use as having the texture of “sticky muesli”. That, he says, makes it attractive for self-builders who may not have the necessary skills to use the more commonplace plaster. It can also be molded into shapes, textures and finishes.

He cites a social housing project in Suffolk, England as providing a good example of the superiority of hemp as a building material. Suffolk Housing Society built the first two hemp houses in England, as part of an 18-unit social housing development, then studied their performance compared to the regularly constructed buildings.

A report was issued in 2002 by the Building Research Establishment (BRE) in regards to the sustainability, economic and environmental differences between the two construction methods. The report’s principal conclusions are that while the hemp homes have far less impact on the environment – they use less energy to build, create less waste and take less fuel to heat – they cost about 10 percent more to build than brick and block houses.

In North America, there are a few hemp houses. In the U.S., the Lakota on the Pine Ridge reservation have constructed a community-based hemp house that was built as a model for sustainable economic redevelopment. The house used hemp and adobe bricks, hemp insulation, and experimented with hemp fiber reinforced cement board.

Hemp house in Ireland
The McCabe house, built by the Oldbuilders Company and Eco Habitats, was the first hemp building to be completed in Ireland. Similar to a standard timber frame house with a mix of hemp and hydraulic lime structured around the timber, it was finished with a lime rendering inside and out. The 42-square-meter building is to be used as office space.

Another hemp demonstration house, which was much more ambitious in nature, is the rural Ontario, Canada home of Kelly Smith and Greg Herriott, the founders of Hempola, a manufacturer of hemp food and body care products. The walls of their spectacular 4,500-square-foot octagonal home north of Toronto are filled with hemp bales in a technique similar to straw bale construction. The floor and ceiling beams of mostly reclaimed wood are stained with hemp oil and the roof is shingled with hemp composite.

With over 120 different projects in the last nine years having used the material in Ireland and over 250 in 16 years in France, this revolutionary but simple material has now come of age. And thus the number of commercially available hemp building products is also increasing.

Washington State University has produced hemp fiberboard, which is lighter, twice as strong, and three times as elastic as wood fiberboard, plus it has sound proofing and pressure isolative characteristics absent from wood fiberboard. The process involves chipping the hemp stalk, bonding it together with resins and glues, and clamping it down into molds under high pressure until it hardens.

A company in Chatham, Ontario called Wellington Polymer Technology Inc. is trying to keep up with demand for its maintenance-free Enviroshake brand roofing product, which resembles cedar shakes. Enviroshake is made from hemp, in combination with recycled materials such as post-industrial plastics and crumb rubber from tires.

Hemp is the main ingredient in a French product called Isochanvre. The manufacturer has developed a method of crystallizing the hemp sap and the resulting product has found its way into numerous building products and materials. Isochanvre is mixed with hydraulic lime and water to bind it together, then packed into timber formwork and left to solidify like concrete.

A number of companies are using hemp in insulation products, due to its high thermal resistance, ability to absorb and release moisture, and lack of mold growth, dust and other pollutants. Thermo-Hemp, from Ecological Building Systems in Ireland, is available in both mats and rolls.

England’s Natural Building Technologies, which is a leader in developing sustainable building materials, has a competing product called Isonat, a high-insulation material made from hemp and recycled cotton fibers treated with inorganic salts to provide fire and pest resistance.

Hemp-based paints have even been created and have proved their superior coating and durability characteristics, although the cost of the oil will prevent any mass marketing of them until political climates allows widespread cultivation of hemp

One hemp enthusiast has estimated that there are 13 broad categories and upwards of 25,000 specific applications for industrial hemp. Having been used for centuries around the world, it’s certainly poised for a come-back in modern housing construction. 

Learn More

Building with Hemp by Steve Allin (Seed Press, 2006)

The Oldbuilders Company (many hemp construction photos)

Rolf Priesnitz is the Publisher of Natural Life Magazine. He also has over 40 years experience in the construction industry.

This article was published in Natural Life Magazine in 2006. It is one of a small number of articles from the magazine that are also available for free on this website. To read more, please subscribe.

     

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