by Cedar Rose

Most people choose straw bale for environmental considerations, wanting to be kinder to the earth and to themselves. Straw bales have low embodied energy; they’re easy and fun to use, while being light on the earth; and they can help create a nurturing home environment.

Earthen plasters share all of those characteristics with straw bale; they are completely compatible mates. The straw provides us with a natural material to keep our homes well-insulated and protected from harsh exterior elements… and similarly, earthen plasters protect our bales with natural techniques which embrace the ancient roots of cultures all over the world. We can achieve strong, durable finishes while being responsible for our personal impact on the planet and the health of our homes.

The current production level of cement and concrete causes serious environmental damage by creating significant amounts of air pollution, water pollution, and damage to the earth from mining operations. Current uses of cement and concrete introduce out-gassing into home environments from additives, curing agents, radioactive materials, and possible radon exposure from aggregate used in the mix. Synthetic stuccos introduce a combination of harmful chemicals which also out-gas. Concrete and synthetic stuccos compromises the health and well-being of our families and our planet.

While concrete in some applications (like foundations and slabs) can be sealed with non-toxic sealers to prevent out-gassing into a home, the use of these sealers for concrete stucco, or the use of synthetic stuccos, is not recommended because it reduces the breath-ability of your straw bale wall system, potentially creating significant moisture-related problems.

Whether we are addressing moisture from condensation, the weather outside, or build-up from the interior of our homes, the ability to dissipate it is important. Moisture can be a problem when it is trapped in the building components by a non-breathable surface.

Straw bales have the ability to breath and dry out, except under extreme conditions, without compromising the bale’s integrity. Cement stuccos can cut down the breathe-ability of the wall system; sealed cement stuccos more so. On the other hand, earthen plasters protect the bales by shedding rain while simultaneously creating a breathable surface which assists the drawing-out of moisture which inevitably enters the wall system, usually in the form of vapor.

Clay molecules are shaped like platelets. When wet, they swell naturally to seal the moistened surface preventing deeper penetration of moisture in towards the bales—unlike concrete, which wicks moisture in toward the bales. The clay platelets also act to draw moisture from within the bales out to the surface to dry. This is an important element of system durability, especially in harsher climates with higher moisture and/or freeze thaw cycles which can wreak havoc with concrete finishes. When time comes for repairs, earthen plasters are much simpler and easier than concrete.

We have found time and time again that, when done right, earthen plasters and finishes are far superior to concrete and synthetic stuccos in many ways. In comparing the complete picture of earthen systems against other stucco systems—including durability, longevity, materials costs (financial and environmental), ease of use and application, availability, long term maintenance, effects on the complete building system, and human health—we have found that hands-down (and hands-on), earthen is the way to go.

A long lasting, durable Earthen finish on a straw bale structure is accomplished by considering the following factors:

House Design. Where does the water go? Think about interior as well as exterior moisture concerns. Climate considerations: Snow? Rain? Winds? Hurricanes? Blizzards? Where do the storms come from? What time of year? How do roof systems come together? How do your walls come together? etc.

Construction details. With consideration of your plaster system, how have you connected your walls, foundation, roof? How do you tie in from one building component to the next? Have you eliminated non-natural and/or non-breathable surfaces that your plasters may come in contact with? Constantly keep in mind how you can “key” your plasters into your structure.

When using earthen plasters, always have your bales stacked with the cut sides to the interior and exterior; don’t have your bales on their sides—this eliminates the best surface area for your plaster to key into.

Plaster Mixes. Your mixes are determined primarily by the type of soils you have available to you, which are unique to each project. Where are the materials from? Do you have an adequate supply to complete your project? What is the clay content? How does the clay respond to moisture? What type of mineral composition is your clay? How “workable” is your clay? How absorptive is it? What is the clay/silt/aggregate content of your dirt?

Additives are often used in earthen plasters: manure, cactus juice, flour paste, ox blood, etc. Fibers are variable as well, including straw, animal fur, cattail fibers, hemp, etc. Each has their own contribution based on their constitutions: proteins, fibers, enzymes, microorganisms, pH, minerals, etc. Your mixes are unique, and additives and combinations of materials are also dependent on the materials at hand. Do many experiments, determine the composition of your mix and determine which will serve each application the best that it can.

Application. It is extremely important to “key into” (that is, have a mechanical bond between) each layer of straw and plaster. Chicken wire, metal lathe, gaps in the bales and loose straw can all contribute to eventual failure of an earthen stucco finish. The air trapped behind the plaster causes its surface to be vulnerable and be one of the first areas to breakdown.

Joints and spaces between bales need to be packed with a damp clay/straw mixture, and kneaded into the bales themselves to prevent loose areas. Deep holes can first be packed with dry straw, and then packed on the surface with damp straw/clay. We like to do this during our first slip spray process. This packing gives the plaster a strong surface to key into and eliminates the problem of plaster from falling off of our wall for lack of mechanical bond.

A common mistake made with earthen plasters is to spray or spread a thicker plaster mix directly on the bales as a first coat; this eliminates a strong bond between the plaster surface and the straw bales. Instead, apply deeply-penetrated slip coat of water and clay first. You can use a stucco sprayer, drywall texture gun or a hand pump sprayer to apply a clay slip coat. Another option is to use a very damp straw/clay mix and “slop” it on the bales. The most effective and least time-consuming is the sprayer or texture gun technique, which provides a consistent coated surface for the plaster.

Let the slip coat dry, then mist the wall and go over it with a wet straw/high-clay/sand scratch coat mix, making sure you are keying into the spray coat well. Whether you apply this by hand or sprayer, use your hands to work it well into your surface coat.

Once you have applied this coat, let it dry. Mist it down again and apply a higher sand mixture that has more structural integrity to it. This layer is used to fill in and shape your walls. Once this coat has dried, moisten again and apply a finish coat.

Between each coat, except for your finish coat, leave your surfaces rough and mist them before applying your next coat. All the while, focus on “keying into” your previous coat, working your plaster layers in well. Cracks in your surfaces are good for keying your next coat into. Avoid cracks in your finish coat, of course. Although earthen plasters take work and time, the end result is incredibly satisfying, beautiful, durable, and contains your own unique personal touch.

The earthen finish coat can be left as is, or oiled or finished with an assortment of natural clay plasters, paints, aliz, or casein finishes with a huge variety of natural pigment colors.

To simplify a fairly complex topic, if you have a strawbale house well designed for the use of earthen plasters and natural finishes, if your mixes achieve a solid, durable, breathable surface, and your application techniques have keyed in and secured your finishes well, you will achieve a durable, long-lasting earthen finish for your strawbale structure. For a plaster system which is natural, easily available, fun, environmentally supportive, and a healthy building material, Earthen Is It!

With over 20 years of experience in natural and holistic building and design, Cedar Rose Guelberth has developed effective natural plastering techniques for straw bale for many climate conditions in a large variety of projects. She teaches and consults throughout the US, Australia, and Canada. She also owns and operates the Building For Health Materials Center, which offers non-toxic building supplies and finishes, appliances, and more. Email [email protected]