There is an old saying, “an adobe without straw is like a person without a soul.” That is not to say that it is impossible to make a good adobe without straw, but speaks of the long-standing relationship between the two materials that is as old as anyone can remember.
Earthen materials and natural fibers have a strong affinity for one another. They preserve and protect one another. In combination with earthen materials, natural fibers such as straw increase tensile strength and help control shrinkage and cracking. The addition of earth or clay to natural fibers brings about a significant increase in fire, water, and insect resistance.
In contrast, natural fibers decay when they come into contact with highly alkaline substances like concrete, especially in hot, humid environments. For that reason natural fibers are not used to reinforce concrete without additives or modifications that neutralize the negative effects of the alkalinity.
Prior to the publication of our book, The Straw Bale House, most of our work with straw consisted of building with it in its bundled and compressed form—i.e., bales. Over the past three years, the beneficial relationship between earth and straw has increasingly captured our imagination. For us, the above statement with a slight twist would be true: “A strawbale building without adobe is like a person without a soul.” The two substances are so compatible that we find it increasingly difficult to conceive of using one material without the other, and in the process have discovered a world of seemingly unending possibilities in the combination of earth and natural fibers.
Our most recent strawbale structures have become earthen/strawbale hybrids. Once the strawbale walls have been stacked, we use traditional English Cob, a wet earthen mix with a high concentration of straw, to fill all the cavities and joints in the bale walls. Cob is like an adobe mix, but with as much straw as the mud mixture can accept before it fails to bind. We often draw on the same material to shape window and door openings, create moldings, or form extensions or shelves that grow from the walls. The same mix of earth and straw has allowed us to create highly curved sculptural interior walls, bancos for seating, as well as free-form fireplaces.
When there are larger spaces in a wall that need to be filled but are too small for a bale to fit, we often choose a mix of loose straw coated with a clay slip. Popularized in Germany over 400 years ago, this is known as Leichtlehm, or “light clay.” This mixture of straw and clay is most commonly packed into slip forms, creating walls that are essentially “rammed straw.” Unlike rammed earth however, it requires no heavy machinery, and less intensive form work. It can be used to create thin exterior or interior walls, insulation and ceiling panels between rafters, load-bearing components between floor joists with second stories, or insulation below adobe floors. The straw/clay provides less insulation than bales, but offers greater flexibility in constructing walls of different widths. Packed into adobe brick forms, straw/clay can also make lightweight blocks.
We often use straw/clay to fill smaller framed wall sections where bales would take up too much space—for example, in south walls with large expanses of glass, or in gabled ends. Above window and door frames is another place we often use this mixture, with one of the most recent examples being a beveled window opening with a rising arch built of split bamboo and filled above with straw/clay.
Our pursuit of more knowledge about this wonderful material brought us into contact with German earth builder Frank Andresen, who widened our world of clay and natural fiber combinations to include clay/wood-chips, clay/sawdust/straw, wattle-and-daub, clay/water-reed/burlap-panels, as well as marvelous natural clay plasters with natural fiber lathing and reinforcement systems.
These combinations of earth and straw have accompanied us over the past few years to Ciudad Obregon, Mexico, a modern agricultural city in the state of Sonora, which is annually overwhelmed with the burning of almost a million acres of straw. Together with Mexican co-workers and friends Teodoro and Emiliano Lopez, and members of a neighborhood called Aves de Castillo, we have been stretching these earth and fiber combinations to new limits for creating ultra-low income housing and shelter. The last structure we completed was a simple rectangular, load-bearing strawbale structure with parapets of straw/clay and a unique flat roof constructed of small wood beams, carrizo (an abundant, bamboo-like reed) and a straw/clay roof coating. This little room (with electricity, no plumbing) cost a respectable $1.75 a square foot when completed—clearly competitive with the cardboard, corrugated asphalt paper, and pallet-wood shacks of the surrounding area. Approximately half of the $1.75 per square foot went for the wood window and door frames. The straw/clay mix on the roof was coated with a mix of white cement, powdered marble (marmolina) and an acrylic waterproofing agent.
Since this building was finished, another small structure built entirely of load-bearing straw/clay bricks has been completed.
Adobe block used for interior walls and bancos (seating) has proven to be an excellent way of providing thermal mass for structures with strawbale walls. Straw bales can be used to create super-efficient passive solar buildings, but they need the addition of adequate thermal mass to store the heat gained during the day from the sun. Several years ago, we built small passive solar strawbale guest house that uses a combination adobe-interior-wall/banco for thermal mass. By using adobe, we were able to create a beautiful, sculptural, curved wall that was quickly constructed and perfectly fit the space while providing the mass we needed to make the building perform as intended.
Earthen plasters have been used extensively in many countries for many centuries. We have found earthen plasters to be an excellent covering for strawbale walls. An almost inseparable bond is formed between the plaster and the bales as the mud mix becomes completely integrated with the straw.
Earthen plasters are breathable, allowing whatever moisture may be in the bale walls to escape. There may be no other building material capable of regulating moisture levels as effectively as earth, which continually absorbs and releases moisture as necessary. Being a flexible, forgiving material, like straw bales, earthen plasters are easier to repair than harder surfaces like cement. They can be applied by hand or machine to the bales directly, without wire reinforcement. We do trim with a chainsaw the “folded” side of the bales to ensure a good bond between the earthen mix and the straw. By screening this loose straw through ¼” hardware screen, we also are able to make all the short-length straw we need to add to the plaster mix.
We typically stabilize the exterior plaster with the gel from the prickly pear cactus, commonly known as nopal in the Southwest; however other materials such as linseed oil, lime, or polymer-based products like concrete bonder can also be used. If more weather resistance is needed, we use one or two coats of lime plaster over the initial coats of mud, in much the same way that German straw/clay buildings are plastered.
Straw and earth also can make a soft, insulating, and beautiful floor which, if done carefully, can be sufficiently durable for most domestic applications. The use of straw/clay between floor joists for a second story floor base also allows the use of an earthen finish floor. Our own experience over the years, coupled with extensive interviews with other builders of earthen floors, has yielded methods of constructing earthen floors that we feel are suitable for most climates and conditions.
A lingering dream of using straw thatch roofs is also beginning to come to fruition. During the past year, we have worked with thatchers from the southern Mexican state of Tlaxcala (where thatching with rye is still in practice), and with English thatcher Aaron Greenwood. The first step in reviving this beautiful art has been securing old long-stemmed varieties of grain that are suitable for thatch roof construction. We are currently in the process of growing out crops of some of these special varieties for use on upcoming projects. Some of this straw will be used in Mexico, while the rest of it will be reserved for a five-week training program that Aaron will be conducting during the summer of 1997.
It’s truly an amazing experience to watch these two materials—straw and clay—combine to create almost every facet of an entire building. By viewing building as a process of combining different-but-complementary materials rather than adhering to a particular building system, we have given ourselves the freedom to create structures that respond to a wide variety of contexts and circumstances. They can be elegant or simple, quick or detailed, inexpensive or costly, and probably most importantly, they can be built from predominantly local materials in whatever combination best matches the local climate. Like the rest of life, building can be much more fulfilling when founded on a good relationship. For us, viewing building as a process of combining earth with natural fibers has led to an unfolding of options and possibilities that would not be open to us if we were to remain simply straw bale builders.