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Hemp - the Ultimate in Sustainable Building - by Chris Counsell Msci

The Truth About Hemp

The Hemp plant is the most versatile crop in the entire plant kingdom and has shown its uses throughout history. It is thought that the use of Hemp for food and fibre began at least 12,000 years ago. The Columbia History of the World says that the oldest relic of human industry dates back to 8,000BC and is, incredibly, a scrap of hemp and from 1000BC to 1883 AD hemp was the largest agricultural crop in the world. The first book was written in China on Hemp paper, and the U.S. was founded on Hemp.

Hemp-680x456Over 25,000 products can be manufactured from hemp, including paper products, foods, building products, fabrics (the original Levi jeans were made from hemp but lasted too long to be commercially viable!), diapers, insulation, carpets and most importantly, its potential as a sustainable, non-polluting biomass energy. Hemp is the number one biomass producer on planet earth: 10 tons per acre in approximately four months. It is a woody plant containing 77% cellulose (Wood produces 60% cellulose). About 6% of United States farmland area put into cultivation for biomass could supply all current demands for oil and gas (from 'Energy Farming in America', by Lynn Osborne).

In the 16th Century Henry VIII encouraged farmers to plant the crop widely to provide supplies for the British Navy. A steady supply of hemp was needed for the construction of battleships and their components. Riggings, pendants, pennants, sails, and flags we all made from hemp fiber. Hemp oil was also used as a sealant on the timber of ships. Hemp paper used for maps, logs, and even Bibles that may have been on board. George Washington was the largest Hemp farmer in the world during the late 1700s and Thomas Jefferson called on farmers to "plant Hemp seed, not tobacco." Hemp was even considered legal tender for almost 200 years in the United States. You could pay taxes with Hemp!

It's easy to be confused about the differences between Hemp and Marijuana. But the more you learn about Hemp, the more you will find what an amazing material it is. Though Hemp and Marijuana are both varieties of the Cannabis sativa plant, smoking Marijuana will make the user high, and smoking Hemp will not.

Hemp contains less than 0.5 percent of the active ingredient THC (delta-9-tetrahydrocannabinol), the substance that gives pot smokers a high. Marijuana plants, on the other hand, contain 10 to 20 percent THC.

So why are we not utilising this amazing material? The simple answer is because of misinformation and persecution of this crop because of its close relation to Marijuana.

Hemp-590x335It is possible to grow hemp in the UK under license, and the industry is steadily expanding.

The Home Office and the Ministry of Agriculture allowed the relaxation of restrictions concerning the cultivation of cannabis sativa in February, 1993, albeit under strict licensing controls.

Hemp made a commercial reappearance in Britain after an absence of thirty two years.

Hemp for Building

Construction products such as medium density fibre board, oriented strand board, and even beams, studs and posts could be made out of hemp. All of these products would be direct replacements for wood based materials and because of hemp's long fibres, the products will be stronger and/or lighter than those made from wood. However the product that I find most interesting is the use of Hemp hurds in lime concretes and renders. Hemp hurd is the inner part of the stalk of a hemp plant after all of the fibres useful for paper and fabric have been taken off. This stalk can then be chipped up and used as an aggregate. This is the most environmentally friendly form of building that is possible for a number of reasons:

  • Hemp is a sustainable aggregate i.e. you can regrow it (unlike sand!)
  • CO2 is fixed during the growing of the hemp and locked up within buildings
  • Lime is burnt at lower temperatures than cement and reabsorbs CO2 whilst curing
  • Hemp does not require any pesticides or herbicides and is one of the fastest growing crops available
  • Hemp aggregate produces insulating concretes and renders meaning that additional insulation is not required

These materials have been used in France for the past 15-20 years and in Ireland for the past couple of years. In England these materials have only really been used in a trial sense but it is an area that is growing quickly. Two buildings were constructed in Haverhill, Suffolk, in association with the local housing association and the BRE.

The thermal resistance of Hemp concretes is between 0.11 and 0.13 W/mK. This means that to achieve U values of 0.32, which is the current standard for wall insulation in building regulations, you would need to cast walls 375mm thick. However in practice the performance of Hemp concretes is better than you would expect from its U values. SAP ratings and U value calculations carried out on the Haverhill houses suggested that the hemp houses should have used significantly more energy than the brick houses, but this was not the case. The data collected using thermal and humidity loggers in the homes revealed that the temperatures maintained in the hemp houses were consistently one or two degrees higher than in the brick houses for the same amount of heat input, that translates into heat savings of between 10% and 20%!thermographic

Note in the pictures above the distinct difference in the heat of the outside of the building, it was -1oC when they were taken.

This indicates that the hemp houses displayed significantly better insulation properties than the conventional U value calculations predicted.

It goes to show that although these indicators are useful they do not give a complete picture in insulation performance.

It was found that the hemp houses cost more to build than the conventional houses but it seems that a lot of that extra cost was because the builders were using new materials that they weren't accustomed to as well as problems with supply of these unusual materials. This is really encouraging because these extra costs should be possible to overcome, through improved working practice and increased economies of scale.

Building Methods

Building with hemp lime concretes is really very simple with a mass pour being done in to shuttering around a basic timber frame building that supports the roof. It is possible to build solid wall structures like this because lime is used as the binder and therefore it is a completely breathable system meaning that damp is not a problem, providing the right plasters and paints are used. Tests carried out by the BRE on the hemp concretes during the Haverhill project found that after 96 hours of constant water spraying it did not penetrate further than 70mm into a 300mm block. Having said that particular attention has to be paid in making sure that hemp is not subject to dampness either due to stocking conditions of the raw material or capillary action in hemp mortars (adequate damp courses if constructing walls or insulating/draining ballast base in concrete should be in place). The qualities of St. Astier natural hydraulic limes are ideal for making hemp mortars. A special binder (Batichanvre) has been prepared for making Hemp mortars. I will outline the main building techniques here but for further details, methods and mix proportions please see St Astier Hemp Construction.

Hemp/Lime Insulating Concrete

Hemp/Lime concrete requires 1 bag of Batichanvre (3 5kg) for every 100 litres of Hemp (comes in 200lt bales) +35-40 litres of water. When constructing a floor slab directly on to soil place 15- 20cm of ballast onto a well levelled and compacted area of soil and then you can lay the Hemp/Lime concrete on top without the need for plastic sheeting. The hemp concrete should be applied in layers of 5 cm. maximum and level with a rake. Compact by tamping gently. To obtain the highest thermal insulation this first layer should not be compacted (this will be suitable in loft insulations where the highest compressive strength is not required). The final layer will be levelled with a straight edge, compacted and floated with a wooden float.
The minimum thickness of the hemp/lime insulating concrete is 8 cm. At this thickness the thermal conductivity of the concrete will be between 0.11 to 0.13 W/m.K depending on the compaction. The compressive strength at 90days will be 1.8 N/mm2. The production is 800 to 850 litres of mortar per m3; of hemp + 10 bags of Batichanvre, depending on compaction. Tiles, wooden floors or carpeting can be laid on a further 4cm of lime concrete screed suitably finished after the total drying of the work (up to 90 days depending on thickness and climatic conditions). Keep area well ventilated and avoid forced drying.

Timber Framed Walls

Fix 2 x 2 wooden battens at the middle of each frame (See diagram below). Then construct shuttering between frames to cast the hemp and lime mix into. Place the mortar in layers of 5-10cm. and compact gently (Compacting too hard will reduce the insulation properties) before next layer. Remember that you must leave room to place final render/plaster (15-20mm). If a plaster or render is going to be applied, movement joints should be used every 3m2. Dosage, mixing ,water addition and production is the same as outlined for hemp/lime concrete.Hemp-700x266


Construct shuttering to allow for the required thickness of the final render/plaster. Insert timbers into the mortar. These should be totally covered by the mortar with a thickness of minimum 7 cm. depending on the size of the timbers used. Place the mortar in layers of 10-15cm. and compact before next layer. Pipes, plugs etc. should be positioned previously or as the work goes on. Shutterings max. 1 m. high. Once completed the first level, fix the second level panels and apply a second layer of mortar, about 25cm. and compact. Only then the lower panel can be removed. The first level mortar has to remain enclosed for min. 30 minutes. Apply same method to the remaining levels.Hemp-450x268

If a plaster or render is going to be applied, movement joints should be used every 9m2. Dosage, mixing ,water addition and production: as per hemp/lime concrete.

External Render and Internal Plasters

Use NHL 2 (Feebly Natural Hydraulic Lime) for base and finishing coat once the structure is completely dry (approx. 60-90 days depending on weather conditions and thickness). Protect external work from adverse weather conditions. Internal plaster should at all times have an NHL 2 base coat before different finishes, if required, are applied (EcoMortar F, fine plasters, lime paints etc.). Maximum areas allowed: 3m2 for wooden frames and 9m2 for walls. On a properly prepared dry background apply a stipple coat as follows: 1 volume of NHL 5 to 2 volumes of coarse sharp sand.

The hemp/lime mortar can be applied wet on wet (after 12 hours or so) by laying on or casting on. The hemp/lime mortar dosage for a natural finish is: 2 bags of NHL 2 + 100 litres of hemp + 60/70 litres of water.

The thickness of the render averages 5cm (up to 8cm.). It is applied in layers of 2-3 cm. at about 20 to 90 minutes interval between each pass. The last coat (2 cm) to be applied 24 - 48 hours after the previous coat. This coat will be floated to achieve the required finish. The average consumption rate is: 27kg of NHL2 + 55 liters of Hemp for 1 m2 of render @ 5cm thickness.
If a decorated finish is required use Batichanvre instead of NHL 2 and proceed as above. The decorative finish (lime wash, Ecomortar, stucco etc) can be applied after a drying period of 45-90 days. Consumption: 38.5kg of Batichanvre + 55 litres of Hemp for 1m2 of render @ 5 cm. thickness. The thermal conductivity of the above renders will be between 0.11 to 0.13 W/mK, depending on the compaction. In all cases the surfaces should not exceed 1 5m2 without movement joints. Walls should have a damp course to avoid moisture transfer by capillary action. The information supplied in this document is for general guidance only. It is recommended that test panels are constructed in all cases.


With its rapid growth and high yields Hemp is one of the most effective forms of Carbon Sink (ie. Carbon absorption) available, even more so than reforesting! It is also the most versatile crop available and can be used for building, foods, textiles, paper, plastics, energy production and much, much more. Being able to use an aggregate for concretes that is not only sustainable, in that it can be regrown and it absorbs and locks up CO2; within buildings, but is also insulating (Hence reducing energy consumption) is nothing short of miraculous.

When you add to this that agriculture within the UK is declining steadily and desperately looking for alternative crops that have high potential revenue the arguments for using Hemp become more and more compelling. The specialist processing and harvesting machinery for Industrial Hemp production is expensive but it is already feasible at current production rates. If this crop was to be grown on a larger scale then it could also become a lot cheaper than modern building methods.

It is this combination of factors that have led me to believe that this is the only building product that can significantly reduce a buildings embodied energy, and significantly reducing ongoing energy use, as well as being sustainable in terms of long term production for future generations.

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