When it comes to choosing an appropriate plaster system for a strawbale, rammed earth, cob or adobe structure, there are many important factors to consider.
The traditional use of mud plasters and renders to coat and protect walls dates back a very long time and is found in almost all regions of the world. Finishing a house with mud plaster when the house itself has been built with earth or straw is a natural, complementary technique. Earth-based plasters often use earth in combination with other natural materials such as wheat straw or cow dung, to to improve the basic qualities of the earth by acting as stabilizers, hardeners, and waterproofers. Even without additives, however, mud plasters and renders can give excellent results provided they are made and applied with skill and care. Earth plasters have many complex qualities and require a proper understanding. Given that understanding they can perform to a very high level, and are durable and attractive. Earth-based plasters are completely compatible with natural materials and building techniques, and the almost universal availability of suitable earth for building gives them a distinct advantage over some modern cement based plasters.
Earth plasters have very specific unique qualities which are extremely well suited to natural building materials.
In terms of breathability earthen plasters not only have excellent vapor permeability (a µ factor of around 8 ie only 8 times the equivalent thickness of air), but also extremely good hygroscopic qualities. What is significant here is not only the amount of moisture that can be absorbed from the air but also the rate of response. In most materials hygroscopic qualities relate to the capillary structure of the material, whereas in earthen plasters, moisture can also be drawn in and held by ionic bonding with the clay particles themselves. Earthen plasters have a much more rapid uptake of moisture from the atmosphere than with other materials such as wood and straw, which take in and release large quantities of moisture but over a much longer period. They can therefore act to protect vulnerable organic materials (and in particular straw and wood) from high levels of relative humidity, when microbial and insect attack can be triggered. Particularly with modern building usage (showers, cooking, indoor living) this can be an important strategy in the control of excess moisture in vulnerable buildings.
In addition to the building benefits of clay, the hygroscopic qualities mean that molds caused by condensation are minimized, and that a relative humidity of 50% -60% is maintained. This is the ideal level for mucus membranes of the human body, and also for the control of dust mites and other organisms which affect human health. Earthen plasters also have very good capillary qualities. They actually have less capillary draw than materials like lightweight brick, and even certain cement products, but more capillary draw than straw or wood. This means that within a strawbale wall they will draw water droplets away from the straw, but not suck in water droplets and hold them against the straw. (It should be noted that when earthen plasters are used externally they do need a limewash or a fine lime plaster skim to protect them against driving rain and snow, depending on levels of exposure. Rain resistance can also be substantially improved by the use of an oil based sealer.
Earthen plasters are flexible in relation to their fiber content. In this sense they are similar to fat lime plasters. Their inherent soft and pliable qualities mean that fibers such as straw, flax, and hair, are able to hold together the plaster without cracking in situations of minor or gradual movement, provided there is sufficient quantity of fiber. This is a significant quality in natural buildings.
Earthen plasters to an even greater extent than fat lime plasters, are easily repaired. Unlike lime, cement and gypsum they are also re-workable.. Because of theirbreathingqualities they can protect more vulnerable parts of structures, and absorb large amounts of moisture, salts and pollutants where these are a danger they can be used sacrificially.
earthen plasters have a very particular aesthetic. Due to the
shrinkage of the clay (micelles ) on drying, the plasters always have
an open texture even when polished. This means that light reflects
and refracts on the surface in way in which there is always variation
and never a gloss sheen.