EARTH PLASTER SYSTEMS

                    

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.

A building that is constructed of earth or straw needs to have a thick layer of some type of protective plaster on the interior and exterior to protect its structural components from the effects of weathering.

The recent resurgence of strawbale and earthen building techniques has seen a number of interesting technological developments in the last couple of decades. One of those “advances” has been the widespread use and acceptance of earth and lime plasters for strawbale structures. The industrial revolution and the portland cement era had implanted many powerful notions within modern builders about the need for a “strong” and impermeable plaster skin for all buildings. When builders started experimenting with strawbale, cement stuccoes were an obvious first step. Now, after more than 25 years of development, most experienced strawbale builders agree that portland cement plasters are an inappropriate choice for buildings of earth and straw.

Cement based plasters are very strong and very brittle. They tend to crack excessively, both due to shrinkage upon curing and structural movement. Cement based plasters are largely hydrophobic, meaning they strongly repel water. At first glance beneficial, keeping water out of a wall, but if moisture does enter a wall, cement plasters will have a tendency to keep the moisture in rather than wicking it out. Cement plasters have a low vapor permeability, they tend to not allow a wall system to breathe, which tends to promote condensation through cracks and air leaks. A wall system that is not breathable results in poor indoor air quality. Cement based plasters are hard to the touch and have poor acoustical properties. Cement based plasters are difficult to repair and impossible to do so without a visible patch.


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.


Breathability


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.


Flexibility


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.


Repairability


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 their“breathing”qualities 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.


Aesthetic


Unpainted 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.


Artesano has developed a highly effective, cost competitive earthen plaster system for strawbale and earthen buildings. We use an exclusive blend of commercially available clays, sands and crusher fines to produce a beautiful, durable plaster that will protect your natural building forever.


      We are available for start to finish application within reasonable driving distance from Boulder Colorado. We also offer a COMPREHENSIVE CONSULTATION SERVICE to help builders from anywhere design and apply a successful  earth plaster system for their project.