No doubt our personal favourite with rendering is the use of earth. Preferably the earth that comes from your footings or foundations. With earth/clay one needs to do a lot of testing of the material. We strongly advovcate building a test wall and for first timers try everything before you go to the trouble of rendering the complete building.
One of the major advantages of strawbale walls is the ability of the walls to exchange vapour. That is, the walls allow the outside air to be heated or cooled as it passes through the mass of the walls before mixing with the interior air. Walls can be finished so that they are sealed to allow varying degrees of vapour permeance. One danger of a completely sealed wall is that if for any reason moisture enters, it will have a very difficult time getting out, leaving the bales susceptible to mold, fungus and decomposition. Therefore, we need to consider plastering walls with natural finishes, such as lime, gypsum or earth.
We strongly recommend that you use earth/clay/chaff with a vapour permant siloxane moisture barrier as a finish.
We would be happy to discuss these mixes with you, and we conduct regular strawbale building workshops using earthen renders. The following mixes can be used as a guide.
• 30-50% clay/sand mixed to a sticky consistency with water and applied directly to the strawbale walls
• allow to dry and apply to a thickness of 12 mm. Note: do not use any netting at this stage or any stage using earthen renders.
Use the same slip mixture and add sand to achieve a mix of 20-30% clay and 80-70% sand mix. This stage requires the addition of chaff or sawdust, depending on your preference. We use Golden Ganmain Chaff, and with our soil type the following mix works very well:
• three buckets of soil
• two buckets of washed river sand
• one bucket of Golden Ganmain Chaff
We apply this second coat to a depth of one inch, or 25 mm. This coat is then allowed to dry throughout before the application of subsequent coats of render. We would then use the same mix as above as a filler coat. The amount required is dependent on how straight you want the walls to be.
Using the same mix as the second coat you are now ready to apply this ‘finishing’ coat. Take your time with this stage, as the better the finish you achieve the easier it will be to apply the lime-putty render. We apply this coat to a depth of 25 mm, or one inch. We now have a total render depth of approximately 50-60 mm.
You should also consider using natural finishes, such as lime wash, lime, casein paint, and calcimine paints. These will all give a natural finish and colour of your choice. There are also a number of organic paints and natural oil paints available. If your design allows for wide eaves or verandahs all round then you should consider the alternative render mixes such as:
• lime plasters using natural fibres like animal hair and plant material as a binder
• mixes with additives such as linseed oil, skim milk and casein glue.
Here are some photographs of our Golden Ganmain Earthen Render Mix: