Straw Bale Building

Straw: A Renewable Resource

Straw, the stalks remaining after the harvest of grain, is a renewable resource, grown annually. Each year, two hundred million tons of straw are under utilized or just wasted in this country alone. Wheat, oats, barley, rice, rye, and flax are all desirable straws for bale walls. Even though the early bale homes used hay for the bales, hay is not recommended because it is leafy and easily eaten by creatures great and small. Straw, tough and fibrous, lasts far longer. The U.S. Department of Agriculture indicate that Americas farmers annually harvest enough straw to build about four million, two thousand square foot homes each year, nearly four times the houses currently constructed (D.O.E.)

Straw has been used in various ways as a construction material for as long as there has been agriculture. Early structures implement straw-clay combinations. The straw provided tensile strength and some insulation value, and gave clay building materials additional structural integrity. Europeans used straw lightly coated with clay slip to infill heavy timber construction. Many examples of both of these techniques survive today. Baled straw was first used in construction over 100 years ago by the settlers of Nebraska. The innovation was a product of newly developed baling machines as well as limited availability of conventional building materials to homesteaders in the Midwest.

Straw bales were used to construct many types of buildings, including schools, barns, houses, corner stores, etc. Some 75+ year-old buildings are still inhabited today, and historic straw bale structures can be found in a variety of climates. Straw is the stalk of any grain plant (oat, wheat, rice, barley, etc.). Straw is high in cellulose, similar to wood, and is therefore not digestible by animals. This is different from hay, which is a food source for many animals. After grain harvest, a small percentage (up to 15%) of stalks can be tilled back into the land to re-supply nitrogen to the soil. The remaining stalks, however, are viewed as a waste product by grain farmers.

Loose straw stalks can be used in landscaping or for animal bedding, though these applications use only limited quantities of the total straw produced. As a result, grain farmers currently burn much of their stalk waste, releasing fine particulates and CO2 (greenhouse gas) into the atmosphere, which contributes to pollution and poor visibility. Increasingly, this practice is being banned. Grain farmers across the country have been working to find more valuable and environmentally sound alternatives to burning. In California, when burning was banned, grain farmers were instrumental in helping to pass a building code for straw bale construction. At the same time, consumers are searching for more natural and sustainable alternatives to standard building materials. As a result, straw bale construction techniques have been enjoying a renaissance.

There are 2 basic types of straw bale construction: loadbearing (or Nebraska style) and non-loadbearing (or infill). Loadbearing construction, as its name implies, utilizes straw bales as large bricks that support all of the building loads.

Non-loadbearing construction utilizes an independent structural system with straw bales filling in between. Straw bale infill construction does not rely on bales to carry any of the building loads (other than the weight of the bales themselves). Since the structural system is an independent element, it can either utilize conventional techniques or be engineered according to load requirements and building codes. The infill system makes obtaining a building permit simpler, since code officials can view the straw as an alternative insulation material. Additionally, since the structural elements are familiar, infill straw bale is less intimidating to builders and contractors not already familiar with this type of construction. The basics of straw bale infill construction are simple to learn and require no expensive tools.

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BENEFITS OF STRAW BALE INFILL

CHALLENGES OF STRAW BALE INFILL

• requires careful detailing to prevent liquid water infiltration
• necessitates educating yourself, the builder, and permitting officials
• hired labor can be expensive, especially for plaster finishes
• requires breathable finishes, usually plasters, which may necessitate research or hiring a consultant
• requires more interaction with building officials in regions that do not currently have building codes for straw bale construction in place

Insulation

One of the aesthetic benefits of straw bale construction is that it creates a thick-wall system. Typical bales are 16 to 18 inches wide, resulting in a finished wall assembly that is 18 to 20 inches wide. Since compacted straw has good insulating properties (R=2.7 per inch), these thick straw bale walls result in highly energy efficient buildings, with R-values of approximately 43 to 48. (Note that walls with R=19 are considered to be “super-insulated”.) Insulation acts to slow the transfer of energy through the exterior walls and roof, and a higher R-value signifies more insulation. The less energy is “lost” through the building envelope, the less heating and cooling equipment operates to provide comfort levels. Straw bale structures, therefore, require substantially less energy to heat and cool than conventionally framed structures. (It should be noted that since most energy loss is through the roof, care should be taken to insulate the roof to at least the same R-value as the walls. In addition, straw bale is most commonly finished with thick plasters on interior and exterior. The plaster acts as a thermal mass, effectively “storing” energy just inside and just outside the wall system. This helps to mediate temperature fluxuations inside, and modifies the temperature differential between interior and exterior, resulting in even greater energy efficiency

Materials for straw bale walls are lower cost than for standard frame construction (typically 1/5 the total cost; versus 1/2 for standard construction); labor costs to infill and plaster walls, however is typically more expensive. This accounts for the many straw bale structures that have been owner-built for a fraction of standard construction costs. As a general rule, a structure with owner or community participation for straw bale placement and plastering will be cheaper to construct than if it were built using standard techniques; a contractor-built structure with all hired labor will be similar in cost to standard construction. Where large savings are guaranteed are in the long-term energy expenditure of heating (or cooling) the building.

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Fire Resistance

Loose straw is highly flammable, however bales of straw are compacted tightly enough that they deprive any would-be flame of needed oxygen. In addition, finishes typically used on straw bale walls (plasters and stuccos) are fire resistant, often used specifically as fire-proofing. In a small-scale fire test (ASTM E-119) performed in New Mexico on a straw bale wall assembly (strawbale panel with plaster on one side and stucco on the other), the panel out-performed most standard types of construction (with the exception of solid wall construction of non-combustible materials, such as stone, adobe, rammed earth, etc.). The panel was subjected to nearly 2,000 degrees F on one side for two hours, after which the surface temperature of the non-heated side of the panel had raised only 10 degrees F. The panel did not fail. The surface burning characteristics were tested for flame spread and smoke development (ASTM E-84). This test also passed code requirements, with a flame spread index of 10, and a smoke developed index of 350

Moisture Issues

By far, the biggest concern with straw bale walls, as with any construction materials in a wet or humid climate, is moisture. High moisture content enables fungi to grow and creates the environment for the bacteria that cause decay in cellulose-based materials. There are two main ways that moisture gets into walls: it infiltrates as a vapor or it flows in as a liquid. Liquid water can be prevented from entering a wall through careful detailing (at wall penetrations and horizontal surfaces). Bales should also be “dry” before applying finishes (moisture content should measure lower than 20%) to eliminate the potential of trapping large amounts of water inside the wall. Airborne vapor is a concern only if it becomes trapped. To prevent water vapor from getting trapped in walls, vapor barriers are eliminated, thus creating walls that “breathe”. Finishes are also selected for their ability to breathe.

Pests

Straw bale provides fewer havens for pests such as insects and vermin than conventional wood framing. Once plastered, any chance of access is eliminated.

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Getting a Building Permit

Obtaining a building permit is generally a concern for those who want to utilize straw bale construction. Several states and counties throughout the U.S. have adopted building code amendments that address straw bale construction. Most building codes however, do not yet specifically address straw bale construction. This does not mean that it is impossible to obtain a building permit for straw bale infill construction, it just means that obtaining a building permit will be a non-standard process. Here are some tips on how to open a constructive dialog with building officials: Educate yourself and understand the ramifications of straw bale construction. The building officials will need to feel comfortable entrusting you with this non-standard construction method. If you do not feel confident with your knowledge, you can hire a professional who is acquainted with straw bale construction techniques to assist you. Start an early dialog with building officials. Find out who deals with issuing permits in your jurisdiction and meet with them to communicate your intentions. If they are not already familiar with strawbale construction, provide them with printed information and additional resources and contacts. (Don’t overload them; building officials are busy people and are less likely to read what you have given them if the pile is daunting.)

Follow up and get a list of concerns. Give the building officials a few weeks to review the information. Follow up and have them itemize, preferably in writing, what their concerns are.

Address each concern expressed by the building officials in a clear, concise, and informed manner, and submit your response to them in writing. Be aware that they may respond with additional concerns for you to address. Again, if you are not confident in your knowledge, hire a professional that is knowledgeable with straw bale construction.

This process could take from 2 weeks to 6 months, depending on the jurisdiction you are building in and the degree to which the permitting office is in inundated with other permits. The most important things to remember are to be patient and that the building official is not your enemy (their job is to make sure structures are safe).

Resources:

Straw Bale Construction, Mortgage and Insurance information Cross section drawing of the straw bale house, by Mark Morgan of Bear Paw Design