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Straw bale building was first developed in America during the 1800's and is a technique that is rapidly gaining popularity today. Building with straw bales can be easy, cheap and fun. It is also a way to make a superbly energy efficient building. Conventional construction methods use vast amounts of energy, producing over half of all greenhouse gasses and involve toxic and pollluting materials. By contrast, straw is a natural material of which we currently produce an excess sufficient to build half a million homes each year. Straw is highly insulative, giving heat savings of up to 75% compared to a conventional modern house. Straw bale walls have excellent load bearing capacity and are quite suitable for two storey houses with all mod-cons.
Straw bales can be used either for load bearing walls (which hold up a roof or next storey) or infill (with separate support eg. a timber frame). In either case the bales are stacked like giant bricks and secured to each other with wooden stakes. These stakes can be hammered through the bales or tied in pairs on opposite sides of the wall. When laying bales on a wall the straws should run horizontally, not vertically otherwise the bales will tend to sink into each other.
It is important that the bales be allowed to breathe to prevent accumulation of moisture and rotting. The bale wall should NOT be placed directly on a (plastic) dampproof course, this can cause accumulation of moisture at the bottom of the wall. Instead any moisture should be allowed to leave the bottom of the wall. An 18" high dry-stone wall base (stem wall) will do this job very well if you are building on the ground. If you are building on a platform / stilts a timber base is usually more appropriate.
Straw bale walls are most commonly rendered. A breathable render can be made from a mix of sand and either lime or clay. A lime render is water resistent and hence suitable for exterior use in wet climates. It is quicker setting and often stronger than clay. A cheaper, lower energy clay render can usually be made using local clay soil (better) and avoids the gloves and goggles required for lime rendering.
Either of these renders can be finished with limewash and / or a natural breathable paint. I like limewash and breathable emulsion indoors and an annually repeated limewash outdoors (a very quick job). Note that cement based render should not be used as it does not breathe and can lead to accumulation of moisture in the wall.
Where walls end, by doors and windows you will need some half bales. These can easily be made by cutting one string, re-tying the two halfs and then repeating for the other string. care should be taken to keep the bale as tightly tied as possible.
A generally useful way to tie two strings (or two ends of one string) together tightly is to tie a loop in one. Then pass the other end through the loop and pull it back on itself before tying off.
To tighten a bale string that has already been tied, get a short length of stick, put it under the string then pull and twist to tighten. tuck under to finish.
This is just slightly harder than falling off a log. Take a bale and place one end on a log, another bale or similar support. Apply pressure to the middle of the bale, for this bottom pressure works very well or for a lighter touch you can just kick it a bit.
If you need to get a patrticularly tight curve you can cut the ends of the bale, between (and parellel to) the strings, and remove the straw from behind one string to chamfer the ends. You'll then probably want to tighten theis string a little.
This way works with bends down to 3'-4' inner radius.
Before plastering it is advisable and very satisfying to trim the bales. Removing any loose straws, 'steps' between neighbouring bales and ridges between folded 'packs' in the bales will all make the plastering process easier and more economical as well as giving a smoother end result.
I generally use a chainsaw or hedge trimmer for this job. A sharp axe or garden shears etc will do the job albeit a bit slower. I have also spoken to people who swear by an angle grinder with metal cutting disc.
Whilst you're doing this job you can start to get quite sculptural. If you're working by hand, switch to a saw. For example, at windows you can make quite a significant curve (reveal) as in the photo below. Just be careful not to cut the strings!
In the photo to the right, the sides of the window were curved off as described above. The plastered form over the window was made using the following method which seemed to work particularly well.
Hessian sacking was stapled, fairly tightly, between the window frame and roof timbers. This was done all across the top of the window on the outside with the top and bottom edges tucked inside. Then from the inside I did the same stapling a foot or so of sack on at a time and stuffing straw in the gap.
When stuffing remove straw carefully from the bale to keep it as tightly packed as possible. At the ends where the form meets the wall simply overlap the hessian by 6" (15cm) or so then plaster over the whole lot.
The Natural Plaster Book