Straw Bale House Construction & Inspection Methods
InspectAPedia^®  -    

• Photographs of a strawbale constructed home show construction details, areas where problems occur, history of repairs, durability characteristics
• Straw bale construction methods, costs, durability, advantages, risks
• Strawbale house inspection and common defects
• Sources of guidebooks and "how-to" books on strawbale construction
• How to determine the age of a building from the construction or framing methods used

Straw bale construction uses bales of straw tied with string or wire as the modular building blocks. Straw bales are stacked vertically in a running bond pattern - each straw bale is placed so that its center is over the end-butt joint of two bales in the course below.

The strawbales are laid either on their base (a wall thickness of about 18-22") or on their sides (a wall thickness of 14-18"). For narrow wall sections the straw bales may need to be cut and re-bound to the required length. Small and irregular wall gaps that do not fit a straw bale may be filled with hand-packed wire-mesh or wire lath forms cut to fit the necessary shape.

"Straw" is defined as the stem portion of a grain plant such as the stalks of barley, oats, rice, rye, or wheat, but without the grain (seeds) or grain "heads" of these plants.

Strawbale walls are normally, reinforced with through-driven or surface-tied wood, bamboo, or steel re-bar rods, and then coated on all sides with a stucco of earth, clay, or a cement mixture, applied either directly to the straw bale wall surface or applied to a "chicken wire" mesh or to clay-coated burlap that has been tacked to the surface of the bales or to the reinforcing bamboo on the wall exterior surfaces.

Strawbale wall plastering may be performed entirely by hand, or the builder may use a textured-paint sprayer to first prime the straw wall, making subsequent plaster coat application easier and providing a better bond. During plastering of the strawbale wall, a "cob" mixture of straw, sand, clay and water may be packed into any small remaining gaps between the bales. The straw bale wall plaster recipe varies by location and available materials. The plaster first coat may be an 80-20 mix of sand and clay with chopped straw added for additional strength. The final plaster coat, hand troweled, may also be an 80-20 sand/clay mix with an addition of 4% lime water. This coating will contain less chopped straw or none at all in order to give a smooth hard finish on the wall. Finish coating with lime plaster or similar materials may be applied depending on the climate and building use. For building durability proper curing of the plastered or stucco coatings and final lime or pigmented coatings is very important as hairline cracks can invite moisture into the structure, destroying it.

Post and beam-constructed straw bale homes use straw bales to fill in the wall spaces between wood, reinforced concrete, or steel posts and beams that form the structural frame for the home. The frame supports the weight of the building roof and upper floors. In post and beam strawbale construction the courses of bales may be pinned together using vertical lengths of bamboo (two to a bale) placed on the inside and outside of the straw bale wall, then tied together using a stiff wire "needle" to force twine through the bales where necessary.

Load-bearing-wall-constructed straw bale homes are built using courses of straw bales that themselves form the supporting wall structure for the building. Strapping over the top course of straw bales is (or should be) carried to connections that pin the wall to the building foundation (if a foundation is provided). Some straw bale homes are built without a masonry foundation, resting the first course of straw bales on compacted earth or gravel.

Our photograph (left) shows the sides of a straw bale built home. The "lumps" visible on the building wall are about the size of a bale of hay or straw - we call this modulo-straw-bale lump size, an obvious indicator of the materials of which this home was constructed.

Some advantages of straw bale construction include

• Low cost of building materials. Strawbale constructed buildings that use straw bales for the principal wall material will spend about ten percent of the total building materials cost on the straw bales themselves.
• Rapid assembly of the building as its building blocks are large
• Ability to use low-skilled labor for home construction
• Reduced heating and cooling costs: high wall insulation or "R" values, keeping the home cool in hot weather or warm in cold climates. A typical 18" wide straw bale wall, using an estimated R 2.7 per inch, provides an insulation value of R48. Depending on the climate where the straw bale home is located, heating or cooling cost savings may be significant. Strawbale walls also provide excellent noise insulation between the interior and exterior, but total noise isolation in a strawbale or any other building will be significantly affected as well by the type of windows and doors installed.
• Use of locally available building materials (straw bales) and low-technology construction methods.
• Reduction in the amount of lumber (and thus trees) used to construct the home compared with conventional wood frame construction.
• Fire rating of strawbale walls: properly constructed and plastered straw bale walls are rated as having a two-hour fire resistance, a figure better than some other building materials.

Some disadvantages of strawbale constructed homes include

• Difficulty in obtaining bank financing in some locations. Fannie Mae and HUD lenders may be willing to finance straw bale built homes in some areas.
• Difficulty in obtaining homeowners' insurance protection, or higher insurance policy costs. While straw bale homes that were properly constructed can meet fire code requirements, some insurers may be reluctant to insure these structures or may charge a premium. Check with Farmers Insurance Group for straw bale insurance home insurance rates.
• Difficulty in obtaining a building permit in locations where local building officials are not familiar with straw bale construction. Conversely, in some areas of the U.S. and in Mexico building codes have been written to include and provide for straw bale construction, such as in portions of Arizona and California.
• Less total construction cost savings than anticipated. Depending on labor costs in the construction location, the cost of applying an earth or cement stucco to both sides of the entire building wall surfaces, generally a manual process, may add finishing costs that are higher than those from other building materials. A review of articles and books on straw bale construction shows that cost comparisons between straw bale construction and wood frame construction range from about the same total cost to strawbale construction costing up to twenty percent more than wood frame. Strawbale construction costs that are not found on frame or masonry constructed buildings include the material and labor for earth plaster or stucco coating of the entire strawbale interior and exterior wall surfaces. According to other strawbale experts, straw bale construction saves about fifteen percent of the total quantity of wood used in a normal wood frame structure.If the foundation for the strawbale house is simply compacted earth, that may replace the usual masonry foundation cost of a building.
• Risk of cracks, settlement, leaks, rot, rodent or vermin and insect infestation in the straw bale walls. (Some strawbale enthusiasts inform us that tightly-packed straw bales are not vulnerable to vermin.) Even well dried straw bales are likely to arrive containing some common grain-infesting insects such as merchant grain beetles and sawtoothed beetles (Oryzaephilus surinamensis), and flat grain beetles (Cryptolestes spp). The degree of insect infestation will depend on the straw source, its moisture, the amount of grain fragments or grain dust contained in the straw, and other factors such as when, where, and for how long the straw may have been left before baling, and the subsequent storage conditions of the bales themselves.
• Real estate property value at resale: straw bale homes may not be as readily accepted in some communities or by some home buyers

This straw home photo (left) shows the interior of part of this two story home. Here, too, when the angle of illumination is just right, it's easy to see the straw bale texture of the home's walls. Of course other straw-bale constructed homes may be parged to a smooth wall surface, hiding this straw bale shape.

How durable are straw-bale built buildings?

We have read assertions that well-constructed straw bale houses in Nebraska have lasted for more than 100 years. But as with any construction method, the life expectancy of a strawbale home will depend on both the quality of its original construction and the quality of maintenance over the life of the building.

The type of straw used to produce bales used in strawbale construction may make a difference in the durability of the building. Straw may be comprised of the stalks of barley, oats, rice, rye, or wheat. But the higher silica levels in rice straw make it easier to dry and more resistant to decomposition from moisture.

In the home shown in these photographs, the strawbale constructed home uses load bearing straw bale walls. The load bearing design may be a significant factor in the appearance of the wall cracks and leaks shown in our straw bale wall damage photographs (below).

Our OPINION is that non-load bearing straw bale construction (in which straw bales are used as infill between the posts of a wood, concrete, or steel post and beam structure) may be less likely to experience these problems. The walls of a load bearing straw bale home must carry not only the weight of the wall material itself but also that of the roof and of the live and dead loads of upper floors in the building. Any condition that increases the chance of cracks in the stucco coating of the straw bale wall increase water, insect, and rodent penetration risks and thus the chances of further rot, settlement, or infestation in the structure.

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