CHAPTER 2
Other Natural Building Materials
Straw bales are only one of many natural building materials available to a home builder. While we have chosen straw bales as our favorite natural material for building exterior walls, we would by no means discourage anybody from investigating and using any number of materials and techniques. It is possible to combine many natural material options into a structure ideally suited to your needs and environment.
A Vast Palette of Materials
The same concerns that have fueled the interest in straw bale building have played a key role in the development and use of a whole array of natural and recycled building options. All of these materials and systems are viable options and worthy of investigation by anyone with an interest in straw bale building. Each choice suits a particular use, climate, budget, and level of experience.We will outline some of the most popular of these materials and methods, giving a very brief introduction to each. There are plenty of resources to help you research further, listed at the end of this chapter.
Cob
Cob is among the oldest and most time-tested building methods. Created by combining mud (soils with an adequate mixture of sand and clay) and straw fibers, cob can be used to build both exterior and interior walls, floors, ovens/fireplaces, benches, and other structural and nonstructural elements.
Environmentally sound, often free (if the site soil is suitable), practical in many climates and conditions, easy (and fun!) to mix and build with, cob can be a wonderful medium. Thick cob walls can be load-bearing, but often cob is used as an infill with wooden framing systems. Cob does not offer a great insulation value, but it does contribute thermal mass, making it a good interior wall material in cold climates and an exterior wall material in warmer climates. Many straw bale homes incorporate cob elements, and it’s so much fun to mix and use that we highly recommend you give it a try!
2.1a: Cob house in the UK.
2.1b: (Inset) sculpting a cob fire-place,
2.2a-b: Adobe blocks (Inset) and earth bricks are simple to use and time tested.
Adobe and Earth Brick
Closely related to cob, adobe and earth bricks are also mixtures of sand/clay soils and natural fibers, but the mixture is put into forms and either sun baked (adobe) or compressed (earth blocks) to create rigid bricks in a variety of sizes. These bricks are often dry stacked, but sometimes laid with a clay mortar.
Adobe and earth bricks have a long and proven history and offer excellent thermal mass properties, as well as low environmental impact and cost. In some areas of North America, adobe building has been commercialized and is relatively popular and accepted by building codes.
Rammed Earth
Another soil-based building method,rammed earth utilizes a wooden formwork system into which a suitable soil is placed and then tamped or rammed to compact it, essentially creating something akin to sedimentary stone. Like the other earth based methods, rammed earth is strong, long-lasting, and has a lengthy history.
As well as providing great thermal mass and strength, low cost, and environmental impact, rammed earth lends itself to both low- and high-tech methods. Some experiments with rammed earth as a foundation system to replace concrete hold promise in northern climates.
Rammed Earth Tires/Earthships
Used car and truck tires can serve as permanent forms for rammed earth, providing a long- lasting, stable, and strong building system. A style of building dubbed “earthships” was developed in the US by Michael Reynolds. Using rammed earth car tires as massive retaining walls, earthships are typically arranged with the north side of the building backed into a hill, or buried under the earth,with the south side exposed for passive solar gain. Earthship designs also tend to incorporate systems for renewable energy, water harvesting, and waste processing that make them very self-sufficient structures. These systems are excellent models for anybody trying to lower their environmental impact, and worthy of investigation.
Rammed earth tires can also be used to create foundations for above-ground housing, like straw bale homes. The tires can be laid continuously to create a full frost wall foundation, or stacked in columns to create pier- style foundations. Often, an owner will be paid to take the tires, and the earth to fill them is free, so this is an option well worth exploring.
Earthbags
Using flood control sacks or sandbags to act as permanent forms for earthen fill, earthbag building is also known as flexible form rammed earth. The polypropylene sacks (used for sandbags and feed bags) are filled with sand or other well-draining soils for foundation building, and with clay soils for above-grade walls. Stacked in running bond (like straw bales), these earthbags can create regular vertical walls or can be stacked to create vaulted or dome-shaped buildings. Developed by Nadir Kalili (for use on the moon!), the system can be used to create inexpensive, environmentally friendly, and very beautiful buildings.
The completed earthbag structure is typically plastered inside and out, protecting the bags from ultraviolet (UV) degradation and wear and tear. As with all earthen construction, earthbag buildings provide excellent thermal mass properties, but not much in the way of insulation value. Earthbag building can be used for interior elements for straw bale homes, or to continue bale-like forms in exterior and landscaping elements.
Cordwood Masonry/Stackwall
While many builders pursuing minimal environmental impacts avoid the use of milled lumber, cordwood masonry construction makes use of “junk wood” that would otherwise be ignored. Firewood, deadfall, logging remains, and even construction scraps can be used, as can trees harvested to thin forests producing lumber. The wood is typically cut into 24-inch lengths and laid transversely in the wall, with mortar filling the spaces on both the interior and exterior. The spaces between the logs in the middle of the wall are insulated with loose fill, often a treated sawdust. The system can create strong, beautiful, load-bearing walls with good insulation and thermal mass properties.
2.3: Rammed-earth building.
It has also often been used in conjunction with straw bale walls, typically to create the lower portion of a wall that then continues with straw bale. Recent developments in the use of earth-based mortars have made the system even more environmentally friendly.
Light Clay-straw
In this building style, loose straw is coated in a thin clay slip and then packed into forms to create a nonstructural insulation. Popular for retrofitting buildings in
