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t a k i n g   g r e e n   t o   t h e   e x t r e m e

 

OVERVIEW OF STRAWBALE CONSTRUCTION

overview
the basics
benefits & challenges
performance characteristics
common concerns
additional resources
related articles

OVERVIEW

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. Strawbales 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 strawbale structures can be found in a variety of climates: from hot to cold, and from dry to humid. (Click "Institutional Barriers to Straw Bale Construction" on DCAT website to find additional information and photos of historical strawbale structures.)

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 strawbale construction. At the same time, consumers are searching for more natural and sustainable alternatives to standard building materials. As a result, strawbale construction techniques have been enjoying a renaissance.

There are 2 basic types of strawbale construction: loadbearing (or Nebraska style) and non-loadbearing (or infill). Loadbearing construction, as its name implies, utilizes strawbales as large bricks that support all of the building loads. Non-loadbearing construction utilizes an independent structural system with strawbales filling in between. This fact sheet will address only strawbale infill (non-loadbearing) techniques. (For additional information on loadbearing construction,

Strawbale 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 strawbale is less intimidating to builders and contractors not already familiar with this type of construction. The basics of strawbale infill construction are simple to learn and require no expensive tools.

THE BASICS

 

THE PROCESS

  1. All structural components are constructed (including foundation, walls, floors, and roof).

  2. Interior walls (non-structural) may be constructed. This may also occur after strawbales are in place.

  3. Roof is dried-in and finish roofing is installed.

  4. Strawbales are installed between structural elements. Any penetrations in strawbale walls are created during infill process, including window and door frames.

  5. Any remaining interior walls are constructed.

  6. Electrical and plumbing rough-in are completed and inspected.

  7. Exterior natural plaster is applied, except final coat.

  8. Interior natural plaster is applied, except final coat.

  9. Windows and doors are installed.

  10. Interior and Exterior trim is completed.

  11. Any drywall is installed, taped, and floated. Tape is applied at connections between drywall interior plaster.

  12. Finish plaster is applied on exterior and interior.

  13. All remaining finish elements are completed.

Project-specific elements are scheduled as appropriate.

SOME DETAILS

As shown in the sketches to the left, structural elements can be located within strawbale walls, or can be to one side (interior or exterior). If posts are within the walls, strawbales are notched around the posts as the bales are infilled. If the structure is timber framed, often the wood will be left exposed on the interior, with bales wrapping the structure on the exterior. This gives a clean break for plasterwork, and provides warm wood elements to contrast plaster surfaces.

Interior walls can be typical wood framing or can be hand-sculpted from cob. Wood framed walls may be drywalled, or infilled with a mixture of loose straw coated with clay slip.

Strawbale walls must be protected by water-resistant but breathable finishes, such as natural plaster. These plasters are earthen based or lime based and lend themselves well to strawbale construction. Cement-based surface finishes are not recommended for use on strawbale walls in wet or humid climates. Cements are brittle and eventually form cracks that allow water to penetrate. Since cements are not breathable, they do not allow this moisture to transpire back out at the same rate. Even if other finish materials will be used (such as wood siding), it is recommended to first coat the strawbales with at least one layer of natural earth or lime plaster.

BENEFITS & CHALLENGES

BENEFITS OF STRAWBALE INFILL

  • high insulating qualities (approximately R=42)

  • high sound absorption coefficient

  • simple, easy-to-learn construction techniques

  • structure can utilize standard construction techniques

  • inexpensive material that is usually available locally

  • natural and completely biodegradable material

  • renewable material, needing only a single growth season (with grain head harvested as cash crop)

  • requires very little energy to produce

  • diverts farming waste material

  • aesthetics of a thick-walled building with large window sills

CHALLENGES OF STRAWBALE INFILL

  • requires careful detailing to prevent liquid water infiltration

  • requires breathable finishes, usually natural plasters, which may necessitate research or hiring a consultant

  • necessitates educating yourself, the builder, and permitting officials

  • requires more interaction with building officials in regions that do not currently have building codes for strawbale construction in place

  • hired labor can be expensive, especially for plaster finishes

PERFORMANCE

Insulation
One of the aesthetic benefits of strawbale construction is that it creates a thick-wall system. Typical bales available in the Mid-Atlantic region 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 strawbale 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. Strawbale 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. This can easily be done by insulating the roof with strawbales as well.) In addition, strawbale 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.

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 strawbale 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 strawbale 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.

COMMON CONCERNS

Moisture Issues
By far, the biggest concern with strawbale 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 (for example, earth plasters, lime plasters, natural paints, etc.).

Relative Costs
Materials for strawbale 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 strawbale 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 strawbale 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.

Pest Resistance
Straw contains no nutritional value for pests, thus minimizing concern for infestation. In general, normal termite precautions are recommended to protect structural elements. Though rodents do not have a convenient cavity wall in which to nest, additional care should be taken that walls are completely covered with plaster to provide a physical barrier against rodents.

Installing Electrical
Electrical systems can be installed quite simply. Wiring can be standard type NM or UF. Wiring is pressed between joints of bales or attached flush to the face of the bale wall with 6" landscaping staples. (If wiring is face-mounted, plaster must be a minimum of 1-1/4" thick.) Conduit may be installed for future wiring additions. Junction boxes are attached directly to structural elements or are screw fastened to wooden wedges which are then driven into the bales.

Getting a Building Permit
Obtaining a building permit is generally a concern for those who want to utilize strawbale construction. Several states and counties throughout the U.S. have adopted building code amendments that address strawbale construction. Most building codes on the East Coast, however, do not yet specifically address strawbale construction. This does not mean that it is impossible to obtain a building permit for strawbale 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 strawbale 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 strawbale 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 strawbale 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 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).

ADDITIONAL RESOURCES

WEBSITES

DCAT
www.dcat.net
Development Center for Appropriate Technology with lots of information on strawbale

The Last Straw Journal
www.thelaststraw.org
Quarterly Newsletter - a must for anyone serious about strawbale

DOE Demonstration Project
www.eere.energy.gov
U.S. Department of Energy article on strawbale construction

Straw Bale Central
www.strawbalecentral.com
general info, books, and additional resources

BOOKS

King, Bruce. Buildings of Earth and Straw: Structural Design for Rammed Earth and Straw-Bale Architecture. White River Junction, VT: Chelsea Green Publishing Company, 1996. ISBN 0-226-23916-0.

Lacinski, Paul; Michel Bergeron. Serious Straw Bale: A Home Construction Guide for All Climates. White River Junction, VT: Chelsea Green Publishing Company, 2000. ISBN 1-890132-64-0.

MacDonald, S.O. and Matts Myhrman. Build It with Bales: A Step-by-Step Guide to Straw Bale Construction. (see www.strawbalecentral.com)

Magwood, Chris; Peter Mack. Straw Bale Building: How to Plan, Design & Build with Straw. Gabriola Island, BC, Canada: New Society Publishers, 2000. ISBN 0-86571-403-7.

Steen, Athena Swentzell and Bill Steen. The Beauty of Straw Bale Homes. White River Junction, VT: Chelsea Green Publishing Company, 2000. ISBN 1-890132-77-2.

Thompson, Kim; Jennifer Corson, Michelle Nokken, Chris Watts, and Ken Wilkie. Straw Bale Construction: A Manual for Maritime Regions. Ship Harbour, Nova Scotia: Straw Bale Herbals, 1993. ISBN 0-9680526-0-6.

RELATED ARTICLES

Keeping Strawbale
Walls Dry

Tips on Building Permits for Strawbale

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