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The following answers to common questions about strawbale building are by:
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What are the real benefits of strawbale? The walls speak for themselves. Go and visit a strawbale house or stay in one of the strawbale bed and breakfasts and experience the enduring qualities of strawbale construction at your own pace. Strawbale buildings have a gorgeous feel. The thick walls evoke the presence and security of masonry walls of bygone eras. A timeless quality of building is evoked. The insulation qualities of the strawbale create a calm and cosy environment which responds to today’s need for quiet, energy efficient homes. A super insulated house costs a fraction of a normal home in heating and cooling costs, and is less polluting. We are glad to bring our family up in a strawbale home. The National Research Council of Canada carried out fire safety tests on rendered strawbales and found them to be more resistant to fire than most conventional building materials. Mortar-encased bales passed the smallscale fire test with a maximum temperature rise of only 43°C over four hours. Rendered surface coating withstood temperatures of up to 1010°C for two hours before a small crack developed. In the summer of 1994 in California, a fire storm 90 metres high destroyed a conventional timber-frame home. Nothing of the home or contents was left. The one exception was a recently constructed rendered strawbale patio seat that had absorbed a large amount of heat but did not burn. The owners rebuilt their home with strawbales. Fire is a dangerous foe during the construction phase of strawbale building. Loose straw on the ground, cut bales and stacks of bales are all highly combustible. A tightly-packed, neat, unrendered wall will only burn on the outside, smoulder and then go out due to a lack of oxygen. But be careful of the loose straw around the site during construction. Angle grinders may be the biggest cause of ignition so clean up the site regularly. However, once even the first layer of render is applied you are safe from bale ignition. A fully rendered wall has a wonderfully high fire rating and once insurance companies have been given all the information — see The Straw Bale House — they have no problem granting insurance cover. Bales should be protected from moisture, with moisture barriers at the bottom, top and lower sides of the walls. Over a four-year period, strawbale houses were tested in Canada for humidity, and showed an average relative humidity of 13 per cent despite fluctuations in the environment around the walls. The moisture content of straw must be less than 15 per cent. At 20 per cent, bacteria can attack straw, and it may break down. However, most straw that we have measured has been below ten per cent. If this is the case, then rice, wheat, barley and oats are ideal. Rice straw has some advantages over the other cereal straws. Strawbale walls are best served with generous eaves or verandahs. Keeping the walls as dry as possible will add to the longevity of the building. This needs to be incorporated into the design, while still allowing the sun to enter the building. A carport or garage on the south will protect the southerly wall as well as providing a useful utility area which buffers the building from cold winds. Overhangs to the north should be a compromise between necessary solar gain and light in the building, and some rain protection. If allowed access, termites will nest and travel through straw walls to access door jambs, roof trusses and top-plates. For low activity zones, raised dwarf walls are adequate. For higher activity zones, treatments like Termimesh or Granite Guard or chemicals (please avoid if at all possible) are required as barriers to termite access. In other words, the same precautions as a normal house are required. The best protection against termites is to build on a concrete slab. Penetration through the slab for plumbing ducts can be sleeved with a proprietary collar. A freeboard of 150 millimetres of clean and clear slab edge serves the dual purpose of providing good visual detection of any termite galleries which appear, as well as keeping the bales high out of the ground, avoiding moisture problems. Strawbales provide fewer spaces and havens for pests than conventional wood framing. If a good coat of render is applied and maintained, access for even small bugs is significantly reduced. Few termites like straw compared with the many that like wood. Clean, bright straw has very few mould or allergy problems. Rodents love warm cosy bales of straw. If the render is complete with no holes or cracks to allow them into the interior of the wall, they will not dig a hole through your wall. There are much easier nests available than having to dig through 40 to 50 millimetres of hard render. However, as we discovered, if you take too long before rendering the bales with at least one coat or you cover the bales with black plastic for long periods of time — beware. This is the way to create an ideal environment for monumental rodent constructions and endless activities. How do you get it passed by council?
Any building technology which is not included in the Building Code of Australia requires certification from a qualified structural engineer. Strawbale construction fits into this category. Strawbale construction lends itself to community involvement. It is fun to do and encourages group activity. If large families, volunteers and do-it-yourself groups get together and raise some walls, then strawbale construction will become considerably cheaper than any other type of building. However, if you require skilled tradespeople, then you should expect to pay the same price as, or more than, a standard, brick veneer home. Some examples of low-cost, strawbale buildings exist. These may be simple buildings, built with voluntary labour, and materials being obtained as cheaply as possible, perhaps even free. They certainly attract a lot of interest. However, if the aim is to build a 20 square family home, with bathroom and ensuite, complying with the Building Code of Australia, it may not be possible to achieve the low prices that sound so wonderful. The walls of a house may represent 20 per cent of the cost of a building. If strawbale walls were free the saving on the cost of a house, all other things being equal, would be 20 per cent. They are not free. Sure the straw is cheap. Rendering, however, is not. The work is hard, and if you are paying someone else to do it, it is not a cheap wall to build. If you are budgeting for a strawbale house, talk to someone who has built a similar type of house, with the same level of finish as you require, and built by the same method. If you are going to build the house yourself, then talk to other owner-builders, just as you would talk to former clients of a professional builder you are considering engaging. What is their experience? Where strawbale building represents savings is in the running cost. Heating and cooling is perhaps 25 per cent of that of a conventionally built house. Over the life of a building, this gives savings of tens of thousands of dollars. Who knows what the cost of heating will be in the years ahead? Environmentally this means tonnes fewer Greenhouse emissions are created by a strawbale home. Cost is such a variable in strawbale construction. Strawbale buildings constructed by professional builders are generally quoted at under ten per cent, to more than ten per cent, the cost of a conventional building. Savings are then made due to lower heating and cooling costs. Real savings are made by the use of non-skilled labour with construction efforts that don’t require muscle. Thus all people (mum, dad, kids, extended family and friends) are able to be involved. The involvement of many people is one of the joys of this method of construction. A strawbale house is a delight to build, a delight to behold and a very, very fine place in which to live. How long will the building last? There are many fine, solid strawbale buildings still in good condition in the USA from the late 1800s. This can be used as a guide to the possible life of strawbale buildings. How do you build with strawbales? The loadbearing method is one in which the strawbales themselves support the roof load. A hip roof to distribute the roof load evenly onto the walls is ideal for this building method. The amount of carpentry in the wall frame is reduced. It is easier to make gracefully splayed windows openings with this method. You should consider the placement of door and window openings at the design stage, because there is differential settlement between these areas and full height straw wall sections, which will require adjustment of the top-plate. We have designed one loadbearing building, Country Gates cottage. The cottage looks like a traditional Australian country house with verandahs all around, making the building look at home in the environment.
From a practical point of view, it is very handy to have the roof on before the bales are delivered to the site. This provides a large undercover area to store the bales and to work on them. A tarpaulin can be hung from the frame to protect the bale walls from rain for the few weeks it takes between stacking bales and rendering. With this system rain has not been a problem during the construction. Posts are made up of pine studs, sheathed in plywood, called bucks. This provides a very strong frame, which braces the wall. These are installed around door and window openings. They support a beam which picks up the roof load or floor joists in two storey construction. This method can be seen in The Straw Bale House (page 260) where it is compared to the loadbearing method. Alternatively, a steel frame can be used. The bucks also provide a strong fixing point for curtains and picture rails. Straw is the dry, dead ends left after a grain harvest. A suitable range of crops include wheat, rice and barley. Hay bales are not at all suitable because they contain seeds. To date we have used wheat straw as it is readily available from a reliable source. The moisture content should be under 15 per cent. Generally baling is only done when the straw is dry, otherwise the bales won’t keep and will be unsuitable for most purposes, including building strawbale homes. Unless the bales have been stored in the rain, they will most likely be at this level. The ideal building bale is of a consistent length and firmly packed and tied. It is prudent to select the bales yourself. Strawbale walls can be built on a concrete slab, strip footings or even on poles, or posts with timber or steel joists. The important thing is that the footing is strong enough to support the weight of the buildings and sturdy enough to avoid cracking the brittle, rendered finish. Concrete footings should be at least 150 millimetres above ground level to keep the bales dry. While this always raises questions, there is in fact no difficulty with installing these services in a strawbale building. There is no extra work for the tradespeople because the walls are strawbale and there should be no extra cost. The usual good building practices for location and detailing of wet areas should apply. What can I use for the internal walls? Any material can be used for internal walls. Ideally they will be thin (thus using up less space), insulative (thus decreasing sound transfer between rooms) and should complement the look of the inside surfaces of the outside walls. Attachment to the outer walls can easily be achieved by driving stakes into them and building these stakes into the interior walls. Strawbale walls will be thick — great for kids’ bedrooms or music studios — but too wide for general rooms. Mudbricks, mudbricks on edge, stud walls or light clay bricks (straw coated in mud and set in brick moulds) walls are all viable options. Won’t earthen renders wash away?
Every soil has its own qualities and needs experimentation to find the best recipes and combinations. The use of cement certainly adds stiffness to a rendered wall. If a crack appears in our earthen wall it can be simply repaired with the same earth, re-creating a homogenous wall. Repairing cement renders is not as easy and involves chemical use. An important part of the building design is to protect the walls with a good roof and eaves but this is not so much of a problem if you live in a low-rainfall area. Our buildings have all been built in high rainfall areas. We use a cement render, inside and outside. The cement render allows the wall to breathe, so any moisture in the wall can escape. It is important that the finishing colour does not seal the wall, so conventional paints are unsuitable. Colour can be applied with several methods:
Each of these offers a finish which is maintenance free, without flaking or peeling over time. Alternatively, a limewash paint finish is available in a range of colours. This gives an aged appearance but needs to be re-applied periodically. We do not use cement for render, nor do we recommend it. Earthen renders have a certain warmth and softness. We believe this is an important factor in softening our lives. People often ask for the recipe for render expecting to find a great secret, one that you have been keeping from the world and will only share with them. There is no such recipe. There is a technique. As each soil, clay and sand has its own unique characteristics the actual amounts that will combine to create the desired results will be unique. However, the technique that is used to determine the combination of clay and sand for each render layer is one that, once learned, is applied to all situations. Why don’t you use chicken-wire?
We do use mesh to shape some areas of the strawbale, particularly where we have a complex shape above a door or window such as an arch or where we have a narrow column of straw next to a support post. A thin strip of mesh is also useful in preventing cracking alongside wood where it meets earthen render. We also use Hessian for this purpose. When earthen render is applied directly to bales of straw it can be pushed into the bales easily, thus creating a very strong bond. The straws become the reinforcement. From this strongly attached base the other layers of render can then be applied. How long will the render last? Earthen renders can be rejuvenated by the simple application of another layer. Time between layers depends on the exposure of that particular wall to the elements. It’s a great time to change the colour of you house when you re-render the walls. Just use different clays or add natural colour agents to your clay for a change. Re-rendering can be fun! We currently have a finished wall exposed (no eave) as a sort of test for ourselves and are happy to say it has withstood the high rainfall that our area receives for the last three years. |
The State of Victoria, where most of our buildings have been built, has certain freedoms which have made the issuing of building permits for strawbale construction fairly straightforward. Building permits can be issued by private building surveyors, or municipal building surveyors in, or out of, the area in which the building is to be constructed. For Victoria, this has meant that if a local building surveyor is not willing to grant a permit, the permit can be obtained from an alternative source.
The modified post and beam system uses a timber or steel frame to provide the structure for the building. It is suitable for larger, complex buildings. Building surveyors can be more confident with the modified post and beam system, because the frame can be designed by an engineer using conventional methods. In this system, the bales are merely infill and the main issue for a building surveyor is the durability of the straw.
Earthen renders have been used for a very long time. There are many examples from around the world of earthen renders enjoying a century or more of hanging on. They are available to everyone. They are ideal for owner-builders everywhere.
Chicken-wire is a necessity when using cement renders or when trying to restrain chickens. With cement renders the chicken-wire acts as reinforcement for the render and because you have to attach it to the bales it holds the render in place. Chicken-wire serves no structural purpose with earthen renders.