THE IMPORTANCE OF RENDER
This would have to be the single most important aspect of strawbale building. Technically loadbearing strawbale construction could be described as composite-sandwich construction. The whole structural integrity of your building depends on getting this stage right, as the render skin is also loadbearing. It is also a huge topic in its own right, so all I can do is point out some of the rights and not so rights, and the danger points that can be particularly bothersome if not approached carefully.
The first thing to consider are the render materials. These materials need to be "breathable" - ie. allow the transpiration of water vapour. Don't be fooled - it is almost impossible to make a waterproof skin for your building. I have worked on too many bathrooms that had been "sealed" but had rotted out from the inside. Water is a sneaky critter. It will always get in somehow, but then it can't get out. Then disaster. Not only can the damage reach catastrophic proportions, but it is just plain unhealthy to live in a hermetically sealed unit. So it is best to avoid products like plastic paints, acryllic sealants and so forth. This is expertly explained in an in-depth site you can visit from my website links page.
There is much discussion about whether cement should be used in render mixes. I used to be in the camp that banned its use, until a few issues started to raise their heads. The first issue was that of time. You need to allow four to six weeks between lime render coats to ensure good bonding. Telling someone in the real world of house construction that it could take up to four months to render their house didn't always generate a positive response! A lime/cement render only needs a couple of days between coats.
The second issue is strength. As the render coat gives considerable strength to the bale wall, both inplane and in compression, it makes sense to make it as strong as possible, within reason.
The decider for me was a report on moisture and vapour permeability of renders done by the Canada Mortgage and Housing Corporation (CMHC). They trialed a series of render mixes and coatings for strawbale.
The results are shown below.
The basic conclusion that can be drawn from the figures is that a render mix with a small amount of cement is more water resistant than straight lime, but is still significantly vapour permeable. Add to that the superior strength and speed of application, and I was persuaded. One other comment about cement - Portland cement is simply a combination of limestone and clay, crushed, ground and fired - these are the same ingredients used in "natural" renders. No production of any of these materials is "environmentally friendly". Limestone is dug out of huge quarries, and the sheer volume of native timbers burnt in the production of lime putty needs to be seen to be believed. In fact there is more embodied energy used in the production of lime putty than in Portland cement. I guess my point here is that cement often cops a lot of bad press, but the fact is that it is a valid material in it's own right. To use it or not use it should be based on need or requirement, not because it is "bad".
The basic render mix I use is based on a ratio of about 2.5-3 parts aggregate to 1 part binder. The binder can be cement, lime putty or hydrated lime, even clay, or a combination. Lime putty is superior hydrated lime, but outside of WA it can be difficult to source. However hydrated lime can be soaked in water and aged for a week or two before being used. An example mix would be 9 parts sand, 2 parts lime, 1 part cement. One point to remember in rendering is "weak over strong". Each succeeding layer should be just a bit "weaker" than the one it's covering. It's an easy matter to adjust the ratios to suit.
If you live in an area that has clay, you are in luck. Lime putty works well with clay, and depending on your clay content, and whether you need to cut it with sand, you may need a ratio of lime to clay as low as 1/6 to 1/8. Regardless of whether you are using sand or clay as your base, there are a number of ingredients that can be used in conjunction with the lime to give your render mix different properties. Sugar will help it go cure more quickly, milk powder will make it much harder, glue can make it stick better, and oil or fat can make it more moisture resistant. Any of these materials, or variations of, can be mixed and matched to get the result that you may be looking for. Anyone who has been involved with earth construction would have their own ideas on ingredients, including fresh cow manure and flour paste. The only real problem with all of this is that there are no standard mixes. There are too many variables, such as soil type and so on to allow a definitive set of recipes. You just have to test batches of render and run with the ones that work.
The next aspect to consider is sticking the render onto your walls. Now the art of plastering is a whole world in itself. Books and videos are easily obtained from your local library. You may even be able to gain some work experience with a friendly plasterer, which is the best way to go. What makes rendering strawbale a little different to most other materials is the first coat. This needs to be applied carefully, with special attention paid to certain potential problem areas. One of the reasons that I advocate footings wider than the bales, and that the bales be mounted above floor level is to achieve as good a finish as possible to the joint along the base of the bales. This is a weak point that can be very troublesome if there is an overhang at that point. Put simply, it is a real bugger to get a good finish. Other potential problem areas are internal corners, bale ends and anywhere that the bales meet a different material, such as door frames or the top-plate. Where materials of different expansion and contraction rates meet, there is always the risk of cracking. Careful reinforcing of these joints is needed. One additive to the render mix not mentioned before is reinforcing fibres, such as chopped straw. This gives the render some tensile strength, reducing the incidence of cracking. Fibres in the straw can be used at any point - in fact it is ideal in the first, or scratch coat. Good keying between the coats is essential. If this is not attended to, the risk is delamination of the render layers, particularly in straight lime renders. Once you have that first coat on though, the next coats are just basic plastering practice, so don't be afraid to get advice from professionals.
As I said at the beginning, this is one huge topic, and I'm not about to try and write the book on it! Just be aware that the render is what makes a strawbale building. Pay attention to detail, get advice and experience, don't be afraid to experiment, have fun, and above all, be patient. A rushed job is often a botched job.
Also refer to the websites links for other information on clay slips, alize and other techniques.
And have a look at Spray Rendering for information on render application with air powered spray units.