INTRODUCTION
This is just a summary of the methods and techniques that I use in strawbale construction, and this is where I feel strawbale construction should be at.
First, a point on getting design and construction advice. As someone trying to scratch a semblance of a living in this field, I have to make sure that any advice that I give or work that I do is absolutely spot on, and that it is based on hard evidence and practical testing. This is not always easy, but the effort has to be made. After all, I am playing with someone else's money, not my own. Too often I come across people who have done something once for themselves, and then pass that off as the "right way". They will rattle off pet theories that they read about, or spout "facts" with an almost religious fervour. All well and good until it is put into serious practice. All the theory in the world doesn't mean anything until tested, and testing means that you have to be prepared to modify or change your views on things depending on the test results. The scientific approach means that sometimes things just don't workout as you planned. The upshot of this is to gather as much information as you can, talk to people who have built, but above all, for serious advice talk to someone who takes strawbale seriously.
Now on to the real stuff. There are two types of strawbale construction - "Nebraska", or loadbearing, where the walls take the load of the roof, and infill, where the bales just fill the wall spaces of a structure built to support a roof. At this point I will declare a definite bias - I prefer to (and only) build in loadbearing as it is cheaper than infill, quicker and less complicated, and more in character with the material. I will go so far as to declare that if you can't build your strawbale building as load-bearing, then it probably fails as a strawbale building.
I feel that infill should really be called "strawbale veneer" as in most cases that's really all it is. However, if all else fails, infill has its place where there are extra-large openings, less-than-full-height walls or a pre-existing structure is already available.
In some cases a hybrid structure would suit certain planning needs.
The bale walls can sit on either a strip footing of concrete, stone or other material, on a nearly conventional concrete slab, or on a waffle-pod slab, maybe incorporating strawbales. The base of the bale wall must be at least 100-150mm above ground level outside - preferably more. Alternatively it can be supported by a stumped arrangement. Whichever method is used, effective dampcoursing is essential, and the bales should sit at least 30-50mm above internal floor level on a stem wall. The stem wall can either be of timber, brick, or an upstand as part of a concrete slab, and filled with a loose aggregate for drainage. It should be the same width as the the bales so that the render layers can effectively seal the base of the wall, and carry the render loads through to the footing.
The bales are stacked in a running bond just like bricks. Corners are reinforced with "U" shaped staples. From the third course balecourses are pinned through with 900mm bamboo pins at least 12mm in diameter. They need only be placed at sufficient intervals so as to stop the yet to be tensioned wall from falling over. Once the wall is loaded and tensioned, the internal pinning is no longer structually relevant. Rebar pins or large nails can be placed in the top of the stem wall at corners and openings to hold the bottom course of bales in place. Chicken wire or other reinforcing mesh is not required other than to protect internal and external corners or edges, or door and window reveals as per standard plastering practice.
Window and door box-frames are placed in position as the walls go up. They need to be propped in position until they can be pinned into place. It is preferred that the door and window frames be contained within these structures to save time later. Window box frames can either float freely in the wall, or can be framed down to the stem wall if on the large side. Bales in that case are infilled under the window, but the rest of the structure is treated as loadbearing. Door and window boxes made of sleepers or similar sized timber look good in a thick bale wall, but 4"x2" or 4"x3" timber is sufficient if they are to be rendered into the wall.
The top-plate need only consist of a piece of 4"x2" timber laid flat. The bale tops are strong enough to resist the impression of the the top plate without any trouble, even with the roof loading, and the load is spread down through the bales. The top-plate is tensioned down to the footing by the use of 2.5mm high tensile fencing wire and Gripple in-line wire tensioners. Tie-down points are placed in the footings at construction every 900mm, or middle of every bottom-course bale. Two Gripples - one either side of the wall- are used per tie-down point. Initial tension is taken, then re-tensioning takes place every day or so until the roof is in place. Because the top plate is narrower than the footing, the tie-down wires triangulate with the apex at the top, giving the wall greater lateral strength.
Roofing is straight forward. It is preferable to get the roof in place before rendering commences, as this speeds up the wall compression stage. It should be lightweight, well pitched and preferably hip or dutch-gable for load distribution. Light-weight roof truss systems at 900mm-1200mm centres spread the roof load evenly over the whole structure. Verandahs enhance the livability of any house at a relatively low cost. I have always been a fan of the clean Georgian lines, popularized as "classic Australiana" in the mid to late nineteenth century, and so I am not hung up about having wide eaves. Get the render mix right and you can go any eave overhang you like.
Rendering is the most important aspect of strawbale building. It is this protective layer that will ensure that the building lasts more than a lifetime. Get it wrong and it will produce nothing but headaches. Normally three layers go on. Rendering materials can include cement, lime putty, hydrated lime, sand, earth and clay, and just about any combination of these and other natural materials suitable for earthen constuction. I would recommend a light addition of cement to a basic lime/sand mix. It cures a lot faster, is stronger and more water resistant, and only reduces vapour permiability by a small amount. Do not attempt to seal or water-proof strawbale walls. To do so will eventually create wall failure. The two exceptions to this rule are -
(a)the top of a freestanding garden or courtyard wall,
and
(b)if an exterior wall is particularly exposed to violent wet weather then water proof one side only, only as little as needed, and only if all other avenues fail.
Service installation and fitting out are no more complicated than a "conventional" house. I prefer to go through the walls rather than the slab - easier to access if there are problems. I find it is preferable to build the internal walls of strawbale as this avoids the problem of joining different materials, and is also aesthetically more pleasing. Of course this needs to be tempered when it comes to small rooms, like the bathroom, laundry and toilet.
Now to the cost factor. Strawbale construction is not necessarily synonymous with cheap building. In a standard building wall-costs may make up 20-30% of the total cost. Strawbale can reduce that to about half depending on your labour input. However the rest of the house will cost no more or less than a conventional house unless you start making decisions based on needs, requirements, and sustainability. How big does the house really need to be; do you really need that second/third bathroom/toilet; do you really need all those expensive light/plumbing/electrical fittings; can cheaper materials, be they recycled or natural be incorporated; are we going for the "Better Homes and Garden" look, or do we just want a comfortable and functional home? After all, it is only a house that is being built. It is what you do within the house that will determine whether your life is happy or not, not the house itself.
So that in a nutshell is how I go about building in strawbale. It is all based on experience and experiment as various theories are tried out and either kept, modified or discarded. Read on and draw your own conclusions.