Conga quinto made of HEMPSTONE by Zellform GmbH
Conga quinto made of HEMPSTONE by Zellform GmbH

Building Materials made from hemp

From Hemp you can make wood-like products, made from the compressed woody core of the plant. These can be pressed into fiberboard and other wood building materials. It is interesting to note that hemp wood composites or alternatives are in every way superior to regular wood; termite proof, better for sound and thermal insulation, no paint necessary and, to top it off, much more fire resistant than wood materials. Besides this, hemp materials are sustainable and harvested yearly or semi-yearly.

Or one can mix the woody- hemp core and the hemp fibers with minerals (see below) to obtain a stone texture. 

Of course, we can make anything in between wood and stone depending on the function. For instance, a wall that functions as a supporting wall and also has good sound and temperature insulating properties as well.

Excellent insulation blankets in various thickness are also made from hemp (Germany). Then there are car door- and dashboard panels mixed, layered or sandwiched with polyurethane or plastic. These materials are employed by BMW and Mercedes-Benz. As well as (sound) insulation blankets for under the hood and car carpets.

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Guitars made of HEMPSTONE by Zellform GmbH
Hemp cellulose wood 100x38x10 cm Zellform

John Stahl, our hemp stone expert says the following*: "Our interest in hemp and other alternative fibers for papermaking has led us to considerable research in the use of these same fibers for building materials. There are a number of approaches towards the use of these fibers in building. Some of them are based upon the model of the wooden board, and various strategies of pressed fiber with heat, pressure, and various binders have been tried with a view to creating a substitute for wooden boards and/or panels.

We are pursuing an alternative approach, which seems to us to be a more ecologically efficient solution, since it is closer to the earth, and seems to offer greater advantages and less cost. The basic idea is to use a preparation of hemp hurds and lime to create a kind of cement ("Agstone") which may be used to construct floors and walls which are stronger and more durable than cement or concrete, yet less brittle and much lighter. It also exhibits superior properties of insulation, waterproofing, and fireproofing, and can effectively replace not only the structural elements of a wall, but it can stand alone without any additional exterior or interior wall covering. In addition to offering all of these benefits, the material is easy to work with, and can end up saving considerable expense when compared with traditional building methods.

Snare drum and standing tom made of HEMPSTONE by Zellform GmbH
Drums made of HEMPSTONE by Zellform GmbH

Our research with this approach began with the use of hemp hurds and lime, but since we don't have enough hemp hurds here to work with, we have adapted the idea to the use of other materials: rice straw (a good choice because of the high silica content like hemp hurds) or just about any available chipped up weeds, brush, straw, or other agricultural waste. In the last couple of years we have been implementing these ideas through experimental construction projects.

While there is a wide variation of possible recipes that will all work, it is useful to have a basic understanding of the chemistry involved with the use of lime. When lime (hydrated lime; calcium hydroxide) is used for an exterior coat, it hardens up by simply combining with the carbon dioxide in the air to change back to the limestone from which it was derived (by burning). However, interior lime hardens up by a different chemical reaction: it combines with materials high in silica to harden up into stone. The classic way to do this is to combine lime with clay. This is "Roman cement," and it is really the origin of the whole idea.

Djembe made of HEMPSTONE by Zellform GmbH
Djembe made of HEMPSTONE by Zellform GmbH

Other sources of silica are also used, but there seems to be no agreement about the chemistry involved. Sand is very high in silica, but because of the particle size, it is less significant than clay. However, I have experimented with mixes higher and lower in sand, and my personal experience is that sand is an essential ingredient in the mix. Mixes with little or no sand are just not as hard or durable as mixes made with plenty of sand. Then there is the question of the silica component in the aggregate, the agricultural waste, weeds, chips, or straw. Hemp hurds are high in silica, and that is probably one of the reasons why the hemp hurds plus lime formula has been such a success in France. My personal experience confirms that hemp hurds perform better than just about any other similar aggregate material. Rice straw is also very high in silica, and, while I have not had personal experience using rice straw, I am confident that it would turn out to be an excellent material for this purpose.

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Then there is the sand and clay. This ideally should come altogether if you can find a good subsoil that has a high percentage of clay mixed with sand and silt (no organic matter).

Speaker backing made from hempstone by Zellform GmbH
Speaker backing made from hempstone by Zellform GmbH

Finally, there is Portland cement. Many purists recommend eschewing the use of Portland cement altogether for technical and religious reasons. I will avoid this controversy here except to mention that the technical arguments suggest that too much Portland cement creates problems, but small amounts of cement mixed with lime avoid those problems. We use about 10% cement in our mix because it works: it helps it to set up quickly, and it assists the chemical reaction with the lime. This small a concentration avoids all of the objections to its use other than its supposedly high environmental cost of manufacture, in terms of fossil fuel use, and the release of CO2 into the atmosphere. Anyway, I have experimented with and without cement, and I have found that using only lime for a binder will usually set up hard eventually, but it may take quite a long time to do so. We haven't yet found a really good local source of clay; perhaps if we did we could effectively eliminate cement from our mix altogether.

My latest addition to the recipe is wood ash. The main reason I want to use it is because there is only so much wood ash I can add to my compost, and I want to use it up. Also, lime is highly alkaline and this high pH is required for the chemical reaction to take place, so it can't hurt to use it.

Digeridos made from hempstone by Zellform GmbH
Digeridos made from hempstone by Zellform GmbH

In addition to all of the above, we also use rocks and gravel, especially in the lower courses of our structures. It just makes good sense to use up these rocks where they can do no harm. The more rocks you use up, the less Agstone you will need to use.

Our process allows the use of a much greater quantity of agricultural waste (chips, weeds, or straw) than other methods. We keep changing our recipe, experimenting with new ideas, but the following should give you a very sturdy mix:

For purposes of working out formulas, I always start with one bucket of chips/weeds/straw and call that 10 parts, by volume (we find that formulas measured by volume are much easier to reproduce than formulas by weight). Our mix is as much as 50% chips, weeds, or straw. We use this material because it is freely available and because all those pieces strengthen the mix. We use a random mix, striving for some variation in the size of the pieces. If broken glass is used, that would be part of the 50%.

We used to begin by adding the lime to the chips or weeds, but we have found that it is better to mix up the lime, clay, sand and cement with water and then add the chips or weeds. We recommend three parts of lime (15%), a total sand/clay portion of about five parts (25%), and about two parts Portland cement (10%).

There you have it -- this mix works very well for us, but you can modify it in many directions. You can add chipped up paper and cardboard to extend the mix, but it makes it softer and weaker. You could add more sand and clay, more lime, less cement, broken bottles, floor sweepings, old clothes (shredded), broken toys . . . We pour on site in wooden forms, which are removed the following morning, but you could also press bricks and mortar them together with the above mix, leaving out the chips, weeds, and/or straw.

You can apply a top coat of lime plaster, which is just lime water with fine sand, to smooth out the surface. For a really smooth inner surface, you could add gypsum to the plaster, along with any natural pigments."


Traditionally Linked In The Public Mind To Cannabis, Hemp In Its Industrial Form Is A Valuable Building Material. Sally Smith Considers Its Surprising Virtues:

The rain is relentless and an insidious easterly penetrates every joint. We're standing in Ralph Carpenter's unheated workshop.

Yet neither chill nor damp seeps from the quarry-tiled floor. Our feet are perfectly warm. Touch the bare walls and instead of a moist clamminess there is a comfortable warmth. 

"Hemp," says Carpenter, with confident satisfaction. "It's under the floor tiles, it lines the roof, the walls are made of it. It's simply a superb insulator; warm in winter, cool in summer."

An architect practising from Hartest, a Suffolk village south of Bury St Edmunds, for the past 15 years, he has been pioneering the use of hemp as a building material: the core of the stem a replacement for brick and concrete, the fibres as insulation. He has put it into the fabric of a listed medieval building and built an extension to his own house with it.

Now it is being studied by the Building Research Establishment (BRE) in a comparative housing project. For Carpenter working with hemp has involved seeking out specialist suppliers, relearning old building techniques and devising new ones, understanding its nature and the materials that are employed with it, and even finding the one brand of cement-mixer capable of getting it to the correct consistency.

He lists hemp's benefits: environmentally friendly (no toxic by- products and can be fully recycled); thermally efficient; lower fuel costs; no condensation; absorbs sound; non-flammable.

"Above all it's infinitely renewable because you just grow it. To build our extension took five tonnes and you can crop five tonnes on one hectare in three months. It doesn't need fertiliser. It outgrows weeds and is not prone to pest and disease.

"At the moment at Pounds 550 a square metre, it is between 5 and 10 per cent more expensive than brick; but once the carbon taxes that will have to be paid on conventional building materials clock in, hemp will become more cost-effective," Carpenter says.

For building, the core of the hemp is chopped and treated in a secret petrification process developed in France and mixed on site.

Construction begins with a timber frame of wood of any quality. Uprights are tied in to a brick plinth set in shallow footings. Plywood panels are lightly attached between the uprights to form a mould into which the hemp is tamped.

A day later the panels come off and between is a solid infill, strong enough to hold the timbers firm without bracing. The next lot of panels is attached and the process continues.

The result is so flexible, strong and durable that the footings under the plinth can be shallow, usually 18in deep, half the depth of conventional footings. The trench is lined with sand, stone, brick rubble or any waste building materials. There is no concrete. "We eliminate the need for cement," he explains.

Carpenter's first hemp building, the conversion of a derelict garden shed, was for former Beirut hostage Terry Waite who lives nearby.

The extension to his own house and other small schemes further perfected the processes. Then came the renovation of a wattle and daub building for Bury Town Trust and the discovery that hemp was a more than adequate substitute for traditional daub made with cattle manure.

English Heritage, at first sceptical, was soon convinced as was the specialist who had been collecting her materials from the nearby cattle market. Some panels have been repaired, others entirely replaced, but it is impossible to say which from the texture and appearance.

Hugh Belsey, curator of the Gainsborough Museum in Sudbury, lives there. Initially concerned by wetness in the walls, he says they soon dried out and the ancient structure is warm and dry and not in any way prone to condensation, which can be a problem with renovated old buildings.

"And I have been pleasantly surprised by the heating bills, far lower than I had expected," he says. St Edmundsbury Borough Council, which grant-aided the trust in saving the building, is continuing to develop the use of hemp with Suffolk Housing Society.

A pair of semi-detached houses are being built of hemp alongside another pair, identical in size and design but in conventional brick and block. On the ground floors hemp will also be laid directly over the ground without a membrane and will be used elsewhere for sound and thermal insulation.

Research - monitored by the BRE - will cover financial and environmental costs in construction and in use over a minimum two-year period. By then hemp may have succeeded in winning over the doubters.

Hope for unloved cousin of cannabis

Hemp is in your tea-bags, disposable nappies, jeans and "paper" handkerchieves. Plumbers put it into radiator joints. The oil from the seed is said to be the most nutritious of all oils.

In eastern Europe it is still widely grown for sail-cloth and ropes. But it was banned in the US because of its threat to the paper industry and Britain followed suit.

The convenient argument against it was its narcotic properties, although it has next to none unlike its cousin cannabis. Nonetheless, to grow it today farmers must have a Home Office licence.

Pubdate: Sat, 31 Mar 2001 Source: Financial Times (UK) Copyright: The Financial Times Limited 2001 Author: Sally Smith

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* This information was contributed by our hemp building materials expert John Stahl from the Evanescent Press." For more information please contact