Straw, Cobs, and Rammed Earth: Alternative Building Materials
The exterior wall construction of most houses in the US and other countries is typically comprised of wood or metal framing, packed with insulation and finished with a variety of manufactured materials. However, ancient building methods utilized natural materials, such as straw bale, cob and rammed earth, and they are making a come-back as viable options for sustainable builders. You may not know much about these alternative building materials yet, but over half the world's population currently uses earth to construct their homes.
Straw Bale Houses
Dating back to Africa in the Paleolithic Era, straw bale construction gained popularity in the Midwestern United States around the turn of the 19th century. It's durable against prairie winds, and many of these buildings are still standing. Straw is a waste product from farming and is often burned, which decreases air quality. Instead, straw bales can be recycled and stacked to form load-bearing walls or serve as insulation between the traditional post and beam structures.
Bales of wheat, oats, rye, barley and rice straw are bound with a variety of materials including wood, wire mesh, rebar or bamboo. Typically they are stacked in rows of running bond, which is the most common brick pattern. A raised foundation and moisture barrier are necessary since water is enemy number one for straw. Straw bale homes aren't a good fit for humid climates, however, large roof overhangs help protect the bales during rainstorms. Coats of stucco, plaster, cement mix or clay on the interior and exterior provide further protection.
Unlike straw bale construction, the cob method doesn't involve individual units. Instead, clumps of a mixture of clay, earth, sand, water and long straw fibers are applied by hand. In fact, "cob" is an Old English term that means "lump" or "mass." Wales, England is famous for its cob houses that were built in the 1500s, and many of these structures have survived the test of time.
The traditional labor process sounds fun (think playing in the mud when you were a kid), but it's intense: builders knead the mixture by hand like dough and stomp it with their bare feet as if they are making wine. Thanks to modern construction equipment, the process can be simplified. Cob walls are typically two feet thick, and the sculptural nature of this construction method allows for creative architectural elements including curved and sloped walls as well as arched openings. To see a good example of a cob house, visit Brian Liloia's blog. He documented the process of building his house in the Dancing Rabbit Ecovillage, and it cost only $3,000!
Rammed Earth Houses
Similar to the cob method, rammed earth houses are constructed from a damp soil mixture with specific proportions of clay, sand and gravel. The composition of the soil is important, and you can't use just any dirt. While it's sustainable to use soil from the property where the home will be built, there is the issue of the remaining hole (swimming pool, anyone?). To form the walls, the soil mixture is compressed and tamped into plywood forms. Laborers remove the forms, revealing flat, rectilinear walls. Before the material cures, workers can scrape the surface with wire brushes to add texture or leave the traces of natural wood grain from the formwork. The formwork itself is reusable, adding to this method's sustainable nature.
Rammed earth walls are typically over a foot thick, but they can be much larger: one of the most famous examples is the Great Wall of China! This building strategy works better in wet climates than other earthen methods, and you may apply a sealant for further protection.
If you are building in a cold climate, attach insulation to the exterior. Rammed earth walls have high compression strength and can serve as load-bearing walls. However, they are often reinforced with steel rebar or other materials such as bamboo, and lintels are necessary to span openings.
Think one of these alternative building methods might be right for your future home? Consider the pros and cons that most of them share:
- Unskilled laborers can master techniques with little training.
- Renewable, natural materials are locally available and biodegradable.
- Construction costs are typically much lower than traditional building methods.
- Ideal for those who want to live "off the grid" (i.e. not relying on public utilities).
- Pests such as termites pose less risk for earthen materials than for traditional wood construction.
- Wall thickness provides design opportunities for built-in seating, storage and decorative niches.
- Walls absorb sound and serve as a "thermal mass," which keeps the indoor temperature stable by heating up during the day and then slowly releasing the heat throughout the evening.
- Building code adherence is challenging, as is obtaining financing.
- Finding knowledgeable contractors or builders can prove difficult, and labor is expensive if you don't do it yourself.
- Homes often have an eccentric look, which may be desirable for some, but not everyone: Consider whether you plan to resell in the future.
- Plumbing and electricity lines are difficult to run through thick, solid walls, so they may have to be accommodated in the floor or interior walls.
- Even though these materials are durable, they typically need wide roof overhangs to protect them from the elements, which may limit design options.
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