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The following remains a work in progress and will eventually be used as a basis for a handbook on Light Earth construction in Australia. Most of the verbiage originated in the articles published in The Owner Builder Magazine in 2002 (issues 113, 114 & 115) ©Gary Kruithof 2002, 2006, 2009

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There is a building technique that goes back hundreds of years, a technique that has been well proven, tested and researched, yet has rarely been tried here in Australia. The technique is Light Earth. Light Earth buildings have been documented back to the 16th century (and probably go back a lot further) and buildings over 400 years old are still in service. In the last 30 years in Europe, light earth building has undergone a resurgence in a similar way that mud-brick did here in Australia. As a result, there has been a flurry of research and testing, which means we now have a very clear understanding of the methods, properties, benefits and limitations of this ancient building technique.

In German speaking countries it is called Leichtlehm, Which means light lehm. Unfortunately, there is no good English translation for the word lehm (pronounced something like leem). Some German texts that have been translated substitute loam, and by dictionary definition this is close, but most people in Australia equate loam as gardening soil. The Americans have translated it as clay, but this isn’t right either. Lehm is building dirt; the stuff you find in subsoil layers that contain sands, silts and clays. If you talk about earth walls in Australia, most people know what you are talking about, (see past copies of TOB) so we have chosen to call the system Light Earth. The reason for mentioning all this is that if you go looking on the internet or in the library, you will find very little under light earth, but you will have better luck looking for leichtlehm, light clay, straw clay, chip & clay and a pile of others. Most earth walls are primarily earth with a bit of straw to help control the

mixture, whereas Light Earth is basically bulk material, usually straw, with a bit of clay to hold it together. This creates walls that are relatively light, easy to construct and have good thermal and insulative properties. Buildings of earth and straw are as old as civilisation, and techniques using these two materials can be found all over the world. These two materials are readily accessible in most parts of the world, have low embodied energy, can be worked by unskilled labour and are perfect for creating healthy, energy efficient buildings. How the two materials are combined and worked is quite varied but in most cases clay based earth is the primary material that then has some straw added for tensile strength and shrinkage control. There are earth building techniques that don’t use straw like Rammed earth and Poured earth

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(a relatively new technique) but these usually rely on low clay materials, and use cement instead of (or as well as) clay as the main binder/stabiliser. Mud bricks can be made this way too, but using small amounts of straw is the more traditional method. With all of these methods, the percentage of straw (if used at all) is quite low, with the main ingredient being earth. With Light Earth, the main ingredient is the straw. The most common method of making Light Earth is by using chopped straw that has been soaked in a clay/earth slurry and the mixture is then left to soak over night so that all the dry ingredients soak up a bit of clay. The soaking time is important because it is the clay that will eventually cacoon and protect the straw. The amount of straw will vary but when you can pick up a batch of the mixture with a pitchfork without any slurry dripping away, the mix is ready. Experimentation is usually required to establish the best mix because the type of straw, its moisture content, and the type of clay, will affect the mix. People contemplating any sort of earth wall construction are faced with a choice of many different methods and the choice can be limited by the available materials. As suggested earlier, people considering rammed or poured earth are practically limited to reasonably sandy soils. People looking to good old puddled mud-bricks will be looking for clay contents under 40% so people in high clay areas are usually advised to add sand to their mix to bring the clay content down (they also often add cement which is not always a good idea). Highly reactive clays (those that swell a lot when wet and shrink a lot when dry) are usually avoided in mud brick and cob walls but light earth walls can use reactive clays and high clay soils without any problems. Light Earth has advantages with the straw as well because you are not reliant on bale size as you are with straw-bale construction. The straw can be cut from any shape or size of bale or it doesn’t have to be baled at all, which means large, cheap bales will be fine. You are also not limited to commercially available straw. Indeed a little lateral thinking could find opportunities to utilise local “weeds” or other waste materials. Possibilities include, hemp stalks, reeds, dock, ragwort, Jerusalem artichoke or sunflower stalks, Patterson’s curse, bracken, grape vine prunings, orchard prunings, chopped up cardboard, and many more. The idea is to utilise local materials wherever possible.

Another technique common in some parts of Europe is to use wood chips, wood shavings and sawdust as the main bulk materials and because of the way the walls look when finished, this technique often referred to as “muesli” walls. Mixing the slurry and the straw can be done two ways, by bringing the straw to the slurry, or by bringing the slurry to the straw.

If the earth you are using is nice and close to the building site, (and not too far down) you can mix a slurry in a pit, then progressively add straw until a good mix is achieved, then left to soak over night. Digging holes for septic tanks, foundations, and ponds or nearby dams may present good opportunities for this method.

The other method is more flexible and controllable but takes a little longer. The slurry is prepared in a trough (like an old bathtub) then a layer of chopped straw is laid on a clean surface (a concrete slab or tarpaulin) then covered with a bucket or two of slurry, then more straw, slurry, straw, etc. You now have horizontal layers of straw and clay, so the pile will need to be turned by taking vertical cuts through the pile to ensure a good mix and then left to soak. The main advantage of this method is that the mix can be prepared right next to where it will be needed. Light earth walls are not load bearing. It is a technique designed as an infill between structural posts. There are several ways you can use light clay but the two main methods are either as solid panels or as blocks.

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For panel walls you use temporary formwork to tamp the mix between the structural posts. Using two sets of boards (about 300mm boards seem to be a practical size) you can leap-frog your way up a wall creating full height walls in one session.

If your building project is not ready to lay up full size walls, or you can’t wait the 2-4 months for the walls to dry before you can render, you can make Light Earth blocks. Because light earth weighs about 1/3 of the weight of a normal mud-brick, you can make blocks that are larger and easier to handle. A block 400x200x200 will weigh about 10kg compared to a mud brick 375x250x125 at about 24kg. Light earth is also well suited to hybrid construction (construction that uses a combination of methods) where the higher insulation could be used as the external walls and high mass earth wall used internally to stabilise temperatures. Other people have used light earth in a composite wall with an outer weatherboard skin, a core of light earth then an inner skin of mud-bricks. The weight of the finished light earth walls will vary, depending on the amount of straw that is added. The easiest way of comparing the finished walls is by looking at their density, or weight per cubic metre. Clay has a density around 2000kg/m and straw bales are about 220kg/m (loose straw will weigh less). Obviously by mixing clay and straw, we will end up with something in between. Mixes that end up between 1200kg/m and 300kg/m are considered light earth. Anything more is more like a straw stabilised mud-brick, anything less is just dirty straw. German experimenters tried to create the lightest block possible but at densities lower than 350kg/m , the straw was not adequately protected by the clay and condensation became a real problem. The density also gives an indication of the R-value we can achieve. Basically, the lighter the wall, the higher the R-value but the lower the thermal mass. A typical mud-brick wall 250mm thick will have an R-value of about R-0.4. A Light Earth wall with a density of about 900kg/m , 250mm thick will have an R-value of about R-0.85 and a Light earth wall with a density of 500kg/m will have an R-value of about R-1.8.

The finished light earth is not sufficiently durable to be weatherproof

without a render coat to protect it. Because the walls remain breathable, they also remain slightly porous. This means if

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the walls were subject to driving rain, water would slowly soak in. In this respect, the light earth behaves very much like most other earth walls. One of the great advantages of light earth is that it is a breathable medium. This means that the walls will allow very slow movement of air and (more importantly) moisture so that problems such as condensation and toxic build up (from plastics and chemicals off gassing within the home) are greatly reduced. Therefore, it is important that the render coats are also breathable. Earth based or lime based renders work the best but cement renders should be avoided.

There are two important things to remember when making light earth walls. 1. The straw must be straw not hay. Straw is the stubble left

over from the growing of grain and is mostly the woody stalks. Straw has very little leaf, flower or seed in it, as opposed to hay which includes all these things. Larger critters like mice and bugs will get very little nutrition from straw and will tend to leave it alone particularly once it’s cocooned in clay. Because of the high carbon, low nitrogen content of straw (hay can be quite high in nitrogen) composting will not occur. It is also important that the straw is dry before you start (less than 20% moisture).

2. The earth used must be high in clay, the higher the better (more than 30%). When the clay and straw are mixed, every bit of straw should be coated with the clay slurry. The mix must then be given time to soak so that the outer layers of the straw become saturated with the clay. Since the clay is the glue that holds the whole thing together, proper soaking will make for stronger walls. Also, if all the straw is coated in clay, there is even less chance of attack from rot, mould and bugs.

./$#-+/"&+1/#2$ In most earth building techniques, we are working with the earth in a plastic state. That is, there is enough water mixed with the earth to make the mix workable yet stiff enough to hold its shape, (like plasticine). If we do not add enough water the mix will not hold together, and if we add too much water, the mix will not hold its shape, as it starts to form a thick liquid (this is known as the plastic limits of the mix). As we add a little more liquid, we get a batter like slurry, and this is the mix we are looking for in Light Earth construction. In this state, we still have a uniform mix of sand, silt, clay and water. If we add too much water, and the mix becomes too thin, the individual components of the soil will begin to separate out of the mixture and we will end up with sand at the bottom and dirty water on top. So, if we don’t add enough water, we will not have a pourable liquid, but if we add too much water, the particles of the earth will not remain in suspension (this is known as the liquid limits of the mix). To know the right amount of water that you need to add, you will need to do some experimenting because it will vary depending on the combination of clay, silt and sand you have in your earth. Generally, the more clay you have in the mix, the more water you will need. One traditional method of mixing the slurry is to soak the dry ingredients in a trough, then jump in with oversized boots or bare feet and stomp everything together. I found it easier to use a plaster mixer (like an oversized paint mixer) with an electric drill in an old bathtub.

[Electric tools and water are not good friends, so be careful to keep your tools, leads and plugs dry.] Once the slurry has been made, it should remain usable for a week or so with an occasional drop of water and a quick re-mix. 3&&)#)425

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There are all sorts of things you could add to the mix, but adding things unnecessarily simply adds complexity and expense to what is supposed to be a simple system. Therefore, I would suggest experimenting without additives first, then only adding a minimal amount of other ingredients to obtain a workable brew. A squirt of Dish Washing Detergent can be helpful in breaking down clods of earth. It also seems to make a “creamier” slurry and acts in a similar way to the plasticisers used in conventional mortar. Borax if often added to earth walls in Europe as a mould and insect repellent. In an Australian context, mould and fungus are less of a problem (generalization) but a touch of Borax might be useful as an addition to your termite defences. Bentonite is an amazing material. It is a highly reactive clay that is used (amongst other things) to waterproof dams and irrigation channels. Because it is so reactive (is swells over 12 times when wet) it is not much use in other earth wall techniques but it can successfully be used it Light Earth construction as an additive to sandy or low-clay soils. You will not need much, so start your experiments with small amounts and build up until you find the minimum amount required for a good consistency. In my experiments with some sandy soil, a 2-liter container of dry bentonite was more than enough to treat 360 litres of slurry. Lime is another useful material but would very rarely be used as an additive in the slurry. The main reason for this is that when the slurry is mixed with the straw, it is very important that the soaking period not be rushed to ensure that all the dry ingredients get a protective clay coat. Adding lime may act as a drying agent causing the mix to dry too quickly and unevenly. Lime is best saved for the rendering process. Cement is of little use in Light Earth construction because it simply sets too quickly. There are also complicated issues regarding relative moisture content that would take a separate section to explain, but suffice it to say that unless you skip the soaking period the cement will set before the walls are formed, and if you skip the soaking period, the straw will not be adequately protected. 6'78 Straw is the most common bulk material used in Light Earth construction and because you are not relying on bale size big, cheap round bales will be fine. The main thing to look for here is to get good course Straw, not field hay. The straw must be dry when you start to work with it, otherwise it will not soak up enough clay to bind and protect it. Avoid bales that are mouldy because even if you allow the bales to dry out, there will still be spores left behind to give you trouble. If you are trying to build a chemical free house, you will need to find out where the bales came from because the last thing you would want to do is build with material that is full of pesticides, herbicides and who knows what else. It is also worth noting that most baled straw does contain a little bit of seed. Once you soak the straw and tamp it into place, these seeds usually sprout. It is a little disconcerting to see your walls sprouting green hair, but its nothing to worry about, as the sprouts help suck moisture from inside the wall they actually help the walls to dry more evenly. Once the walls dry out, the sprouts die off and become part of the finished walls.

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The in-situ solid panel method looks similar to rammed earth. Once the straw-clay mix is ready, it is packed between temporary forms that are set on each side of the walls. The mix is then tamped into place and the forms are moved up to the next section. How heavily you tamp the walls will change the properties of the wall you are creating. A heavily tamped wall will be stronger and have a higher thermal mass, but a lower insulation value. Obviously, heavily tamped walls will also use more material. Lightly tamped walls will be weaker but will provide higher acoustic and thermal insulation. In general, to maintain consistency, it is better to lightly tamp the mix to the point that it doesn’t spring back. The density of the mix can be better controlled at the mixing stage (see below). The wall thickness can be varied to suit individual applications. It could be as thin as 90mm, being an infill between standard stud walls, or as thick as 450mm. In most cases, it is easiest to set your framing size as the wall thickness so the formwork can be easily manoeuvred, therefore anything between 90mm and 300mm would seem practical. To give the walls added stiffness, it is common to include some horizontal stiffeners every 600mm or so. These stiffeners can be all sorts of things and include 19X45 timber, garden stakes, thick sticks (20mm+), bamboo, etc. The trick here is to allow for settlement.

When you form solid panel walls, there is usually some settlement of up to 50mm for a 2.4m high wall. This is not shrinkage due to drying, just gravity settling things down. This movement is usually finished in a few days and poses no major problems except that this movement needs to be allowed for with the placement of the horizontal stiffeners. If the stiffeners are fixed to the main frame, the mix may shrink away below them as they settle. For this reason the stiffeners are not fixed to the main frame, but allowed to move vertically as the mix settles. The block method is similar to making mud bricks. The clay-straw mix is gently tamped into moulds and left to sit for a minute before the moulds are removed. The blocks are then left to fully harden. The issues of the density of the mixture are the same for producing blocks as they are for in-situ walls. With in-situ walls, you can avoid the double handling of blocks but you need to have your building project ready to make walls; floor ready, frames ready, roof ready, etc. The advantage of making blocks is that it is not dependant on what stage of building you are up to. It doesn’t even need to be done at the same place you are building. As with the in-situ walls, the block size can be varied but because they are non-loadbearing some thought should be given to a block size that will minimise the amount of cutting required to fit the blocks around the structural frames. 6/#(-)"*0 When I first started playing with this technique, I had a hard time trying to work out the quantities of materials that I would need. Theoretically, the most accurate method would have been to measure everything by weight, but this would not seem very practical on site. The best on site system would be by volume batching. E.g. 6 buckets of straw-1 bucket of clay slurry. Or as a rough guide, 1m of wall of will need about 700 litres of slurry (approximately 1/3 dry ingredients 2/3 water) to 5-6 bales of straw. If you used a volume-batching ratio similar to this, and lightly tamped the mix into place (until it doesn’t spring back), you would be creating walls with a density around 750kg/m . This means a light clay block, 375X250X125mm will weigh less than 11kg, compared to a mud-brick the same size at around 24kg. Advice from those that have used light earth was to initially ignore any pre-set recipes and just play with the mix. Generally, if you make a good clay slurry then keep adding chopped straw (or start with the chopped straw, and keep adding the slurry) until you achieve a homogenous mix (i.e. a mix that holds itself together), you will have the best mix for your materials, whatever that might be. You will need to experiment with this because soil conditions and different types of straw will behave differently. Trust your instincts, because there is definitely something primal (or maybe just leftover from our childhood) about getting your hands dirty. Be sure to keep a record of your experiments so you can establish a reasonable amount of consistency. Experience has shown that doing it this way you should end up with walls between 500-900kg/m .

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</$&+;%$8=+Once you have worked out the best mixture for your materials, and have your framework up, a typical construction sequence would start in mid afternoon. 1. Set the first row of formwork ready for the

following morning. 2. Prepare slurry. Remember we are looking for a

nice free flowing batter like mix. 3. Chop straw. When I experimented with some reeds

and weeds to make a 130mm thick wall, I sat and chopped the straw by hand with an old hacksaw, and it took well over an hour to produce enough for one 3m long wall. My friend up the road used bales of wheat straw, and didn’t bother to chop it at all.

4. Mix the straw and slurry according to your chosen ratio. An interesting advantage of this technique reveals itself here, particularly in multi storey constructions, because there is no need to carry heavy loads up ladders and scaffolds. The dry ingredients can be lifted to where they are needed and mixed on the spot.

5. Take the rest of the evening off! 6. The following morning the mix will have come

together as the slurry gets soaked into the straw. The mix can now be tamped into place and the formwork leap-frogged up the wall.

None of this really takes any longer than other building techniques but the major difference is the reduced weight. Not only are you working with a mixture that is usually less than half the weight of a mud-brick, but because you are working a wet mix, you can lift as much or as little as you want. If you are not in a hurry, building in light-clay is not hard work. 9)")5-)"*+,)*-#+./$#-+;/7750+Light Earth walls are a blend of bulk material like straw or wood chips mixed with an earth slurry. The resulting walls are somewhere between a mud-brick and a straw bale with a balance of both thermal mass and insulation. When they are dry, they form hard, single thickness homogenous walls that are easy to finish and render. A well-made mud-brick wall may not require rendering at all if the base material is already durable. A straw bale wall must be rendered no matter how good the bales are. A Light Earth wall also needs protection with a render because, even if the earth slurry is reasonably durable, the straw or other bulk material can act like a wick, soaking moisture into the wall. The rendering process for Light Earth is very similar to that used in straw bale construction, except you are not trying to apply a hard skin to a soft surface. Applying a hard skin to a hard surface uses a lot less material and reduces the susceptibility to impact damage. Rendering of Light Earth walls should not start until the walls are dry. For monolithic walls this can take from 8-15 weeks or more if you hit a long patch of cold, damp weather. This kind of time delay is sometimes listed as one of the down sides to Light Earth construction, but for most owner builders this kind of time lag should not be a problem as you could be progressing on other parts of the job whilst the walls are drying. Several tips to speed up the drying are to leave out the doors and windows for as long as

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possible so there is a free flow of air inside and outside the house. Never leave the formwork in place as this just traps the moisture, and if seeds sprout, let them grow, as they will be sucking moisture out of the wall for you. If you just can’t wait that long, the block method is for you. Because of their greater surface area Light Earth blocks dry a lot faster than a monolithic wall. If you made blocks as the first part of the building process, they would be dry 6-10 weeks later. By then you could have constructed the floors, wall frames, roof, drainage, etc. Then the blocks can be laid and rendered immediately. Rendering can be achieved a dozen different ways but

the traditional way is in a two to three layer process. The first coat is referred to as a Parging coat. The parging coat is the sin forgiver. It is usually made from the same slurry used in the walls with the addition of chaff and/or sawdust but not nearly as light as the walls. This mix is then applied evenly over the walls to about 10mm, filling in all the gaps, cracks and holes that appeared when you were making the walls. It is important that the parging coat is not applied until the main walls are dry, because this coat is supposed to fill any shrinkage cracks that may occur as the wall dries (which will mostly happen as vertical settlement or a little shrinkage around posts). If there is sufficient bulk material incorporated in the parging mix, shrinkage should not be a problem and no other reinforcing will be necessary. If, however, large cracks do appear, they can simply be moistened and reworked of filled with more mix. Any small cracks can be filled with subsequent render coats. Shrinkage can also be minimized by applying the parging coat with a reasonable amount of force, ensuring there is a good bond between the render and the Light Earth wall. When the parging coat is dry, the finishing coats can begin. Externally, renders based on the same clay used in the walls can work well. They may need the addition of sand to reduce shrinkage cracks. Lime-sand based renders also work well and will maintain the breathability of the walls. Renders do not need to be thick and will only require multiple coats if the first coat cracks too much. In relatively dry climates these render coats will provide adequate weather protection provided they are maintained. In wetter climates a further sealing coat may be required, this could be a simple lime wash, or a linseed-turpentine based sealer, or one of the many commercial products specially designed for earth walls that make the walls water resistant (not water proof) and retain their breathability. Internally, earth based or Gypsum based renders will be all that is required. (Note; internal wet areas should be treated in a similar way to external walls) Before you start rendering, give some thought to the fixtures and fittings. It is often easier to plan for these things early so you don’t have to go chopping into the freshly finished render. Lightweight things like pictures and ornaments can be fixed directly into the walls with good chunky nails or screws. However, anything with a bit of weight needs to be fixed to something more substantial. If a structural post is not in the right place for a particular item (like a kitchen cupboard, or toilet cistern…) you can fix a mounting plate into the wall ready for fixing. Likewise, things like electrical wiring, plumbing pipes, computer cables, etc. You may have to hack in to the Light Earth wall a little to make all these things fit, but don’t worry because the parging coat will hide everything. 1)"&%;5+/"&+&%%$50+It is easy to talk about rendering plain flat walls, but if you consider that every door and window is like a hole that will break the continuity of the render coat, it is worth giving careful thought to how you will finish and weatherproof all these holes in your walls. When installing windows it is good practice to make the window sill protrude past the external wall so that any water running down the window drips off the sill rather than running down the wall. The only problem with this is that if you install the

The term parging has two meanings. In medieval times, a parging coat referred to the first coat applied to a rough surface like a stone wall, to plug up all the gaps and provide a smooth surface for the finishing coats. As the artisans were applying these coats, they often incorporated decorations and patterns particularly around doors and windows. From Elizabethan times the term parging coat had come to mean the decorations worked into the first render coat. In this article, we are using the term in its older sense but by all means take the opportunity to include a little decoration in your walls.

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windows (or just their sills) before the walls are made, the protrusions can get in the way of the formwork. One way around this problem is to use a window profile (Americans call it a buck), which is basically an outer frame that can be installed as you go, with the windows fitted later on. You would normally make the profile opening 10-15mm bigger than you need for the window so that if any adjustments are necessary, you can easily pack and fill around the window to keep things straight and level. Another advantage of this method is that the windows can be kept out of harms way until they are needed and all the structural work is done. A little lateral thinking and we can make the sides of the profile loadbearing then suddenly we dramatically reduce the number of posts we require. !"5'7/#)%"+The R-value of a Light Earth wall depends on its density and the wall thickness. The figures we are using here come from research done in Germany as presented by Volhard (1), that allows us to predict the insulation value of an earth wall based on its density (weight per cubic metre). From the chart below, an average wall at 800kg/m should rate at around R4.0 per 1000mm thickness. Therefore, an external wall 250mm thick would have a rating of R1.0 (a wall at 500kg/m , 250mm thick will be around R1.8). The insulation value of your walls is important but should not be looked at in isolation. It is the thermal performance of the whole house that is important. If we are going to use insulation in our houses (and we should) it is sensible to use it where it is most needed and in most homes this means the roof. The heat gained in summer and lost in winter is mainly through the roof (25-35%) then the walls (15-25%) then the floor (10-20%). If you have many single glazed windows, these can be responsible for up to 35% of your heat loss as well. So you can see there is little point in having super insulated walls without looking at the rest of the house in the same way. With the resurgence of Light Earth construction in Europe and more recently in the USA, there have been people who have sought to create the lightest wall possible, looking for densities around 300kg/m in order to increase the R-value. In some cases this has caused walls to fail through internal rotting. But this demonstrates a fundamental misunderstanding of the medium, as it has been shown that this only happens when too much bulk material is forced into the mix (way beyond the point where it holds together), which means the bulk material is not protected by the clay anymore and/or the walls were sealed before they were dry. As stated previously, the best option is to ignore the R-value and let your materials tell you what to do. Experiment with the bulk material and the slurry and when you find a brew that has plenty of bulk, but can still be picked up in a clump, you will have a mix between 500-900kg/m and an average wall 250-300mm thick will have an average R-value around R1.5. There are those who would waste a lot of time searching for a one-size-fits-all solution, but of course, there is no such thing. I don’t want to present Light Earth in that way, but rather to simply add it to the repertoire of owner-builder friendly techniques that are healthy, energy efficient and inexpensive.

1. Volhard, F. Leichtlehmbau. Karlsruhe 1983. 2. Minke, G. Earth Construction Handbook. Witpress 2000

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