Building With Straw: Sustainable Design

Along with architecture, I have a huge interest in sustainability and sustainable design. When studying this type of building design, you'll find that the possibilities are endless. People have built homes out of adobe, soda cans, car tires, shipping containers (there will be a whole separate blog about this later), and even straw bales. I became interested in straw bale building when I was in high school because it was so controversial. But I decided i wanted/needed o learn more about it, and I decided to do my senior project back in 2011 on it. I investigated straw bales building again in college for a non architectural course, and if given the right site and climate I would design and build one in a second. 

Straw bale construction is a form of sustainable construction that has the potential to change the way we build homes in the US. The use of straw as a building material has many environmental benefits such as reduced C02 emissions, greater thermal performance, and it is a sustainable resource. Straw bale construction has the ability to transform the building industry and social norms about what a building can be.The first straw bale structures and buildings emerged in the United States in the Sand Hills of West Nebraska. The pioneer group who started this movement used the straw bales like giant building blocks. They built their entire town, everything from churches, homes and storage areas out of the straw bales (Minke, Mahlke, 2005). The availability of the straw and ease of construction made this a superior building material for that area in that time. Using straw bales as a building material has continued to spread into the present time as we have become more aware of the impacts our actions have on the environment.

Using straw bales as a building material is a clear choice for those who are interested in sustainability and the environment. This is because using straw bales has many positive environmental impacts. One of the biggest environmental impacts that using straw bales has is that it reduces the amount of C02 released into the atmosphere each year. Straw is the byproduct leftover after harvesting grains. In regions where grain is grown, the straw has become an unwanted waste product because unlike hay, it cannot be used as animal feed (Steen, 1994). Straw also takes a long time to decompose naturally, so farmers across the region has taken to burning the hay to clear it off their land. In California alone, over one million tons of straw is burned each year after the hay is harvested. The amount of C02 produced annually in California by the burning of hay was compared to the amount of C02 produced annually by all of the state’s power plants by the California Agricultural Magazine. They published that the estimated 1 million tons of straw that California burns each year produces 56,000 tons of C02 into the atmosphere. The total amount of C02 released into the atmosphere by all the power plants in California combined only totaled to 25,000 tons of C02 (Steen, 1994).  The fact that the once annual burning of straw in California produces over twice as much C02 as all of the power plants in California do in a year combined prompted the states Air Resources Board to initiate legislation to ban this process (Steen, 1994). Straw bale construction can potentially lower the production of carbon monoxides by thousands of tons a year, which would in turn help control global warming and atmospheric deterioration. In regards to its use as a building material according to Matt Myhrman, author of Out of Bale “If all the straw left in the United States after the harvest of major grains was baled instead of burned, five million 2,000 square foot houses could be built every year”(Myrhamn). 

Another major environmental benefit that using Straw Bales as a building material has is that is greatly improves the R-Value rating of the exterior walls of a building. An R-Value is a measurement of the capacity of an insulating material to resist heat flow. The higher the R-value, the greater the insulating power of the material. A traditionally build home in the United States is made using wood framed construction. A typical 2x6 framed wall with cellulose insulation, house warp an exterior sheathing has an R-Value between 15 and 20.  Compared to a typical straw bale wall, that is 23 inches thick and finished with a form of earthen plaster on each side (interior and exterior) has an R-Value between 30 and 45, sometimes up to 50 (Minke, Mahlke, 2005).Because a Straw Bale wall has a much high R-Value than a traditional wood framed wall, it allows for greater thermal performance of the wall system.  This increased thermal performance increases energy savings because the building itself is able to better retain heat in the winter and disperse it in the warmer months (Steen, 1994). The mass of the straw bale walls also for efficient passive solar heating and cooling designs and allows for natural ventilation.

Using straw as a building material is also beneficial to the environment because in theory it is a resource that can perpetuate itself forever. Straw is the byproduct of the grains and hay that we are already growing. It is considered a renewable resource because it can be grown and harvested in less than a year (Minke, Mahlke, 2005). Compared to lumber, which is a natural resource as well, but it takes a much longer time to regrow to be used again, and when it gone it limits the earths ability to clean the C02 from the air. Straw is also a great building resource because it has the ability to grow in regions where the climate inhibits the growth of timber. Such regions would include China, Russia and arid regions such as Mexico (Minke, Mahlke, 2005).This makes it a practical building choice for areas with low quality land.

A straw bale by definition is a mass of straw that is compressed and bound into rectangular blocks (King, Ashhheim, 2006). They are bound with a twine that can be made of polypropylene, or wire. Bales come in two common sizes, three string and two string. They each have different linear measurements in height, width and length. A three string bale is 14-17 inches high, 23 inches wide, 32-47 inches long and weigh between 75 and 100 pounds (Steen, 1994).  A two string bale is 14-16 inches high, 18 inches wide, 35-40 inches long and weigh about 50 pounds (Steen, 1994). When used in building a bale is laid flat with its width parallel to the ground plane so the bale string is not exposed. When building with straw bales there are two different types of wall assemblies, load bearing and non-loadbearing. More “designed” straw bale homes built by architects and contractors are built in the non-loadbearing method (King, Ashhheim, 2006).  In this method the straw bales are used strictly as insulation infill within a post and beam structural frame. This way of building is generally more flexible to varying architectural styles. It also allows for multiple stories because the loads are carried by the post and beam construction. A load bearing wall assembly is generally limited to two stories in straw bale construction. Another advantage of building a non-loadbearing wall system is that if a portion of the wall ever becomes damaged, it can easily be replaced because it holds no structural bearing. On the other hand building a load bearing straw bale wall system also has its advantages. A load bearing system is often much faster to erect than a post and beam system. (Minke, Mahlke, 2005). They’re speedy construction has made them also quite popular for disaster relief shelters. One of the biggest advantages if that a load bearing system preforms more effectively with greater ductility and energy absorption than a post and beam system under seismic loading. The vertical load on the wall stabilizes it from overturning and increases the shear strength of the assembly (King, Ashhheim, 2006). A method used in both types of wall assemblies that helps brace and add shear strength to the bale walls is pinning. Pinning can be done internally in the wall or externally with steel rebar or bamboo pins. Rebar pins are typically cemented in part of the foundation as a start and are added through the bales as the wall grows in height. (Minke, Mahlke, 2005). Bamboo pins can also be used in the same way. Externally, pins are matched parallel to each other on each side of the bales and are tied through the bales with twine (King, Ashhheim, 2006).  In order to build with straw bales they must also be pre-compressed. Straw bales have a natural tendency to settle over time because of small interior spaces that occur when the bales are formed (King, Ashhheim, 2006). For example, an eight foot high wall will settle between 2 and 3 inches over time under its own its weight. Builders will often pre-compress the bales to avoid settling cracks that will appear in the plaster after the wall is finished. Pre-compression also strengthens the walls structural performance. Pre-compression can be done in several different ways depending on the project. The most common way is to use tension rods attached from the foundation, through the bales and then the top plate/beam running across the top of the beams. Or tension straps that are formed through the foundation and wrapped around the bale wall and tightened uniformly (Minke, Mahlke, 2005).

After the bale wall system is completed it is finished on its interior and exterior faces with a coating of plaster. The plaster serves several purposes in a straw bale wall system. The purpose of the plaster coating is to protect the wall from damage, rain and moisture, air infiltration, insects and fire (King, Ashhheim, 2006).The plaster coating is essential to protecting the bales from moisture because when the straw is exposed to moisture it can begin to rot, which compromises it structural integrity. The plaster coating also prevents air movement through the bale walls, thus prevents vapor laden air from entering as well. Naturally, straw does not attract many insects because it is not a good food source for them or other animals because the grains have been removed. But unprotected straw can be damaged by animals and insects for grain or warmth in cold weather. The plaster provides a physical barrier between them and the straw preventing it from being damaged (King, Ashhheim, 2006) The climate of the region where the straw bale structure is being built also plays a role in the type of plaster chosen. In dry arid climates the plaster will need little maintenance, but in regions where it rains more often and the humidity is high the plaster might need to be touched up or added to depending on if it wears off or not. Plaster also adds to the structural strength of the wall, giving it shear strength and increasing its ability to resist loads (King, Ashhheim, 2006).

Straw bale building has transformed from a small grassroots movement in Northern Nebraska to a worldwide building movement. As we have become more aware of our impacts on the world, the benefits of building with straw have been explored and proven. Building with straw bales is a sustainable form of construction that has the potential to transform the building industry and in turn reduce its environmental impact worldwide. Building anything out of straw might seem laughable to some, but as our world continues to change and advance it would be laughable to not employ a building method that could potentially save our planet.

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Sources

"Earth Structures (Europe) Ltd." Earth Structures (Europe) Ltd. N.p., n.d. Web. 16 Nov. 2013.

King, Bruce, and Mark Aschheim. Design of Straw Bale Buildings: The State of the Art. San Rafael, CA: Green Building, 2006. Print.

Minke, Gernot, and Friedemann Mahlke. Building with Straw: Design and Technology of a Sustainable Architecture. Basel: Birkhäuser, 2005. Print.

Rohrer, Rex, and Lorian Moore. "Straw Bale, Cob, Earth Plaster - Silver Seed Farms, LLC." Silver Seed Farms, LLC. N.p., n.d. Web. 16 Nov. 2013.

Steen, Athena Swentzell. The Straw Bale House. White River Junction, VT: Chelsea Green Pub., 1994. Print.