Perfect Plaster

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Builders of straw bale, adobe, cob and other types of natural homes use earthen plasters for interior and exterior walls, usually applying the plaster in two or three layers. The first coat or two levels out the wall surface. The last coat, or finish coat, provides a smooth, suede-like finish and color. When the finish plaster has dried, many builders apply an alis (pronounced al-lee ), a paint made from powdered clay mixed with earthen pigments, to give walls a wash of dramatic color.

"Earth plasters make the most beautiful walls I have ever seen," Meagan says. "The colors are gorgeous and incredibly varied."

Materials for an earthen plaster often can be gathered from the building site itself — for example, from the foundation excavation — making this plaster an inexpensive material with low embodied energy. (Embodied energy is the total energy required to extract, manufacture and transport a material.) Even if they're not available on site, clay-rich dirt and sand often can he imported from a nearby field or gravel yard, ensuring the lowest embodied energy within the entire family of plasters and stuccos. The small scar that results from extracting dirt for an earthen plaster can be filled easily and replanted, or convened to a small pond, erasing all sign of human activity. Ease of reclamation, combined with its low embodied energy makes earthen plaster by far the most environmentally benign of all wall finishes. And earthen plasters are perfect for people who want to avoid using petrochemicals and other synthetic chemical paints.

Earthen plasters are easy to repair and maintain. If cracks develop in a finished wall, you simply wet the wall and smear on more finish coat, smoothing out the surface and feathering the edges. Alis can be applied to cover up the repair work. Using alis instead of manufactured paints for wall color also spares individuals from exposure to harmful chemicals.

If a home is built with adequate roof overhangs to protect walls from heavy rains, earthen plasters can be used in a wide variety of climates, even some rather harsh ones. Ideally suited to natural home building, earthen plasters allow walls to breathe and permit water vapor that enters a wall (for example, through cracks around window or door openings) to evaporate. This characteristic helps protect earth and straw in a wall's interior from moisture. Water accumulation can cause straw bale walls to mildew and eventually decompose, and earthen walls to deteriorate and collapse.

Although earthen plasters are nontoxic, environmentally friendly and easy to make, they're not always permitted in local construction codes. Local building-code officials may be reluctant to give their approval, especially for exterior applications. However, providing code officials in your jurisdiction with published information on the durability of earthen plasters (see " Resources ," Page 69) and designing a home to protect the walls from weather may convince them of the merits of this readily available, eco-friendly material.

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