Building with hemp A report from France
1 rue Pétion, 75011 Paris, France
Houses have always been built out of what was around mostly stones and lumber. But where stones and trees were scarce, chopped straw mixed with a soil, rich in clay, was a common material. Modern building with hemp is very different from this old technique, yet it too allows for building with what grows around you. (At least in a country like France, where hemp is a legal crop).
Hemp, when grown for fiber, is sown very densely, so as to grow into branchless slender stalks that so resemble straw that French farmers call them hemp straw ("paille de chanvre"). After harvest, the long fibers that make up to about 25% of the hemp stalks are removed. In some parts of the world, like China, these fibers are still spun and woven into cloth. In France, the machinery presently used to remove the fibers from the stalk does not allow processing of the fibers into a thread strong enough to be woven into cloth.
Hemp is still grown for its fiber but today, in France, these are entirely bought by paper making companies (mostly by de Mauduy, a French company now belonging to Kimberley Clarke). Added to ordinary wood pulp, hemp fibers make "specialty paper" particularly pliable and resistant, for banknotes or cigarette paper.
The strong fibers are what hemp has always been grown for. Yet after the fibers have been removed from the stalk, there remains about 75% of the plant. This bulky by-product, known as hurd ("chènevotte" in French) looks like fine wood chips, only much lighter. Hemp hurds are extremely absorbent: they will soak up about five times their own weight in moisture. They are commercialised in France by the hemp growers co-op "La Chanvrire de l'Aube", as bedding for livestock under the brand name "Aubiose". Aubiose is used, in particular, for race horses, who will have nothing but the best. The most minute hurd particles are molded into pellets and commercialized as cat litter. These were first made by La Chanvrire de l'Aube, who commercializes them as "Biochat" in France, or "All Hugro" in Germany. Since hemp growing became legal in England in 1993, cats there can enjoy the same product, made out of British hemp, known as "Posh Paws". Hemp hurds are not only very absorbent, they are also uncommonly rich in silica, a chemical compound naturally occurring as sand or flint. The high mineral content of hemp hurds brings us naturally to one of their most interesting applications: building. Hemp hurds mixed with lime undergo a process of mineralisation, sometimes referred to as "petrification" (changing into stone).
In France, where houses have been built with hemp hurds as the main material (apart from the wooden structure), there are slight differences in technique. But the basic process is the same: hemp hurds are mixed with natural lime (not cement !) and water. Sometimes plaster of Paris (pure gypsum) or sand is added. The mixing can be done in a cement mixer and at this stage the resulting mix does look a bit like cement. It can be poured like cement, likewise it hardens and becomes mold and insect resistant. And yet it retains some of the virtues proper to plant matter: the resulting material, which becomes a lighter, tawny color when dry, has a texture vaguely reminiscent of cork. It is many times lighter than cement (which workers fully appreciate) and it offers both thermal and sonic insulation. The mixture sets in a matter of hours, while the process of "petrification" continues.
The mix can be poured as a floor, or between sheets of plywood (which will be removed a few hours later) for walls. Here one material replaces several layers of conventional building materials: bricks or cement, vapor barrier, insulation, and plaster board. All that is needed, inside as well as outside, is a whitewash finish (with or without pigments added). Alternatively, for interior use, the look of the material can be preserved with a simple waxing or varnishing, which brings out the cork-like structure of the material.
Can the basic process of petrification of hemp hurds be improved? This is where people working in this field disagree. The hemp farmers co-op that initially perfected this technique (La Chanvrire de l'Aube) registered a patent for a process that coats the hurds with silica. The prepared hurds, meant for building, are commercialized under the brand name "Canobiote". "Isochanvre" is another brand name for a similar product, prepared in another hemp-growing area (near Le Mans). Canobiote and Isochanvre look identical, though the latter is almost twice as expensive. Isochanvre costs about 800 FF per cubic meter, Canobiote about 500 FF. France Périer, the woman who developed Isochanvre, initially worked for La Chanvrière de l'Aube, promoting Canobiote (and then left to start her own business). She insists that Isochanvre is a better product, though it is not clear exactly in what way.
Finally there are those like Yves Khün who think that regular hemp hurds, untreated, just as nature made them, are as perfect as can be for the petrification process to best take place provided the right kind of lime or plaster of Paris, is used. He calls this technique "Canosmose".
This former solo dancer with the prestigious Bjart company, now an entrepreneur who works with his architect wife, maintains that coating the hurds with silica makes them less absorbent, which in turn interferes with the petrification process. He therefore prefers to use the hurds as they are, and points out that they are also much cheaper that way (only 270 FF per cubic meter).
Whatever form of hurd is used, it certainly provides a very attractive building material, one that is both healthy for people to live in and environmentally friendly.
© 1994 Michka
Some useful addresses in France:
La Chanvrière de l'Aube(Canobiote)
Rue du Général de Gaulle, 10200 Bar-sur-Aube
Phone: 25271274, Fax: 25273548
Le Verger, 72260 René
Phone: 43974518, Fax: 43976544
Yves Khün (Canosmose)
Ferme La Thuilière, 25560 Monfroc