Straw Bale Basics
Buidling with straw bales
The primary ways to build with straw bales include structural straw bale, post & beam with straw bale infill, modified system (combination of post & beam with structural straw), & standard frame construction with straw bale infill. Straw is also added to many other natural construction methods, such as adobe, cob & straw slip wall. Straw construction can easily be combined with other natural building techniques and materials.
Straw structures provide superior insulation and reduce heat loss due to air infiltration. Since straw is a waste product that under normal conditions would be burned or decompose, locking this material in a wall provides carbon capture of the plants used to make the bales. Straw is available locally all over the country and requires minimal transportation which reduces fossil fuel use. Straw bales are a very low embodied energy material. Using straw for construction is also a great way to avoid sending building materials to the land fill, all the remaining straw materials can be scattered on the property.
Straw bale structures are considered "masonry construction" and are typically 1 to 2 hour fire rated, they are far more fire resistant than other residential construction methods. As there are virtually no gaps in the wall system and no nutritional value, they are not an attractive home for vermin.
Straw is vunerable to water damage. Walls should be set above grade at least 12". Plaster work should provide complete enclosure of the straw to protect from water infiltration. For the best performance of the exterior membrane he walls should be protected by generous overhangs or porches. No vapor barrier is used with the wall system; it is detrimental to the performance of the wall.
Scheduling construction with straw is dependant on the availability of the straw bales regionally. Most construction can begin in the spring with the foundation, structure and roof. Straw bale raising starts as soon as the bales arrives - usually late summer or early fall. As soon as the walls are complete the plastering work can begin. Interior work proceeds through the winter. Most projects are able to be completed in 9 to 15 months depending on size.
Beyond its beauty, building with straw is one of the most eco-friendly construction methods available.
See how to build it
The straw bale
Straw is the waste material from growing crops such as wheat, barley, oats, flax & rice. Any rectangular bale type can be used for construction. The bales should be primarily stalks with few seeds and need to be compressed tightly (enough to lift by one string and not fall apart). Bale ties can be either wire or bale twine. Sizes vary from two string (about 40 pounds, 18" x 14" x 36"), three string (about 80 pounds, 24" x 16" x 42") and jumbo (to heavy to handle by hand, 30" x 21" x 72"). We prefer to use the smallest size - two string. The insulation value is somewhere around R-45 depending on the density and quality of construction.
Altering bale sizes
Although it is eaiser to keep the bales whole, changing the size and shape of the bales is frequently needed. It is a reasonalby simple process to re-tie custom bale sizes and shapes. First measure &/or mark the size or shape on the bale. It is then re-tied intact. Once it is secure the original ties are removed. Gently pull the new custom bale from the original. It takes a bit of practice at first, but quickly becomes a simple process.
Bales can be stacked in any orientation - flat, on the side, or up on end depending on the desired thickness of the wall and the construction method. For structural straw and post & beam construction stacking flat or on edge is used, and for infill frame construction on end is prefered.
For stacking flat or on edge we recommend pining every second course with 60" long, 1/2" diameter bamboo stakes. Rebar is an alternative. Courses are built up row by row with overlapping joints - like bricks. By staggering and pinning the bales the wall gains stability and integrity. The straw wall should also be tied into to the posts & beams. For structural straw bale construction an extra step is needed, the bale wall must be compressed prior to plastering.
The easiest way to plaster the wall is directly to the bales; extra reinforcing mesh or lath is usually not needed except at the connection to the frame or where a specific shape is needed. Correctly mixed plaster sticks very well to the rough surface of the bales.
Plasters can be of several types. We prefer to use a natural type with clay, sand, chopped straw & lime for the first coat and with a finish coat of lime & sand plaster. Completely earthen plasters mixes with clay, straw and sand can be used but are usually a bit softer. Another option is a Portland cement based plaster or the addition of some proportion of cement to an earthen mix. Many plaster combinations are possible and should be suited to region and use. Plastic or synthetic plasters do not work with straw bales.
Plaster application can be done in several ways - the simplest is by hand. It can be applied in the traditional manner using trowels. A spray application is also used: this is the fastest and provides strong adhesion to the bale wall as the force of the spray pushes the plaster beyond the surface of the bale. We use this techniqe for larger projects. Plastering the walls is the most time consuming part of straw bale construction.
Other than the use of straw bales these structures have most of the construction features of conventional construction and the same infrastructure. The other building components are not significantly impacted by the use of straw for the exterior walls. Tnd the costs for these other components and the labor needed for their installation remains the same as for more typical construction. The overall cost of using straw will vary with the level of participation from the owner. With an owner builder doing a significant portion of the labor the cost will be reduced, with a turn key structure the cost will increase perhaps 10% overall. The savings in heating & cooling will quickly offset any initial cost above other forms of construction.
The life span of a straw bale building is considerably longer than a conventionally built stick framed structure. With minimal maintenance these buildings will last as long as other traditional forms of construction, the longest surviving bale buildings area as old as the bale machine (about 100 years).
Not many tools are required for straw bale construction and most of the work can be done by hand. Bale twine or wire is needed for re-tying. A bale needle (long metal bar with a slot or two for twine and one bend end as a handle - easy to make tool) is also used for re-tiying bales and stiching walls. A small electric chain saw is also helpful and can be used to trim the bales and create niches and sculptural details. A string trimmer is also a great tool for creative bale shaping. A small saw for cutting bamboo, and a heavy hammer for the bamboo reinforcing. A "pursuader" (a very large wooden mallet) is used to help align the bale wall.
Building with straw is a very easy process - perfect for the amateur builder. Experienced conventional building crews can learn the process quickly and many natural builders give workshops or classes and are available for assisting with construction. Laura Intscher, principal architect for Secret Base Design teaches several classes a year, as well as giving lectures.
Workshops
Seven Generations Natural Builders