Straw: The Next Great Building Material?
When the idea of building houses out of straw bales began gaining popularity about ten years ago, you could hardly mention the idea without generating a plethora of jokes about the three little pigs. Today—hundreds of houses later—straw is being taken a little more seriously. Books on the subject are selling like hotcakes, and even code officials in some areas are accepting this building system.
While straw-bale construction is where most of the interest lies today, the use of straw in panelized construction systems will likely prove to be more significant for the building industry in the long term. This article takes an in-depth look at straw as a building material, including both straw-bale building systems and building products produced from straw.
The Straw Resource Base
Straw is what’s left over when grains, such as wheat, rice, barley, oats, and rye, are harvested. About 140 million tons (128 million tonnes) of straw are produced each year in North America (see Table 1).
Straw is appealing as a building material for several reasons. First, in areas of grain production, straw is inexpensive. Second, the quality of lumber is dropping, prices are unpredictable, and some suggest future supplies may be limited. Third, because straw is a secondary waste material from grain production, its embodied energy should be fairly low. Fourth, in many areas straw is still burned in fields, producing significant air pollution. In California more than 1 million tons of rice straw were burned each fall in the early 1990s, generating an estimated 56,000 tons (51,000 tonnes) of carbon monoxide annually—twice that produced from all of the state’s power plants! Regulations to ban straw burning are being implemented in many parts of the country, both to reduce air pollution and to reduce the risk of accidents that can occur when shifting winds blow smoke over highways.
In some areas most straw is tilled back into the soil. While straw provides few nutrients to the soil, it does add organic matter and helps aerate the soil. Rollie Sears, a wheat specialist at the Kansas State Agronomy Department, is concerned that the state’s agricultural soils would suffer if all of the straw was harvested. “Taking the straw away would eventually have some pretty significant consequences,” he said. Sears believes that, with careful management, intermittent harvesting of the straw could be done without harm.
There is also evidence that too much straw may not be good for soil. In an article in the Fall 1994 issue of The Last Straw, Ted Butchart of the Greenfire Institute stated that straw is decomposed primarily by fungi and that too much straw in the soil will throw off the balance between soil bacteria and fungi, reducing soil fertility.
The most direct way to use straw in building is through straw-bale construction.
During grain harvest, a baler compresses straw into rectangular bales tied with either two or three wires or polypropylene strings.
There are two primary ways to build with straw bales. In load-bearing straw-bale construction, bales are stacked and reinforced to provide structural walls that carry the roof load. With in-fill straw-bale construction, a wood, metal, or masonry structural frame supports the roof, and bales stacked to provide non-structural insulating walls. With either alternative, the bale walls are plastered or stuccoed on both the interior and exterior.
Load-bearing straw-bale construction.
This is often referred to as the Nebraska style of straw-bale construction because it was used in the sand hills of Nebraska in the late 1800s and early 1900s.
In this part of Nebraska there were few trees for timber-frame or stick-built homes, and the sod was not suitable for the more prevalent vernacular building technique. The development of mechanical balers offered a building material from the primary local resource: grasses. Though straw- or hay-bale construction had many advantages over sod construction, fewer than 100 such houses are known to have been built in Nebraska.
Designers of load-bearing straw-bale structures recommend very simple square or rectangular buildings with hip roofs to distribute the roof load as equally as possible on all of the walls. Buildings are usually limited to one story, and a relatively small number of windows and doors are distributed fairly evenly around the building to prevent differential settling of walls.
In a typical load-bearing design, bales are stacked on a poured concrete stem wall that extends about six inches (150 mm) above the floor slab. Some builders prefer three-wire bales because they are wider than two-wire bales and the walls more stable. Wooden frames are installed for windows and doors as the layers of bales are installed.
As bale walls are erected, steel, wood or bamboo pins are used to join the bales and reinforce the wall system. Sections of threaded rod usually extend from the concrete stem wall to the top of the wall, and bales are installed over them.
These threaded rods are bolted through a wide top plate and tightened down after the roof is installed. Pre-compressing the walls in this manner will minimize further settling after the roof is installed.
In-fill straw-bale construction.
While many straw-bale purists prefer load-bearing or Nebraska-style bale construction, which minimizes wood use, using bales as in-fill walls in post-and-beam buildings offers several advantages. Because a wood, steel, or masonry/concrete structural system carries the roof load, less reinforcement of the bale walls is needed, and bales can be stacked on edge if desired. If the straw bales help support posts, smaller framing members can be used than is common with timber-frame construction. During construction, the roof can be finished before erecting bale walls, so rain can be kept off the bales.
In-fill straw-bale designs also permit greater design flexibility: irregular and gable roof designs, taller 11⁄2- and 2-story buildings, complex floor plans, and different amounts of glazing on different orientations. This last feature is an important advantage if passive solar heating is to be used—and plastered straw-bale walls are ideal for passive solar, because they insulate extremely well and the interior plaster provides superb thermal mass.
The structural post-and-beam frame can go on either the inside or outside of the bale walls. With a timber frame many people like the wood exposed on the inside of the walls. Another common approach is to build a simple post-and-beam frame with 4”x4” (100 x 100 mm) posts positioned at the outside of the straw-bale walls. As straw bales are stacked, they are notched around the wooden frame, where they provide lateral bracing—corner bracing may not be required. Steel frames and masonry-block or poured-concrete columns can also be used.
Other straw-bale construction approaches.
There are a number of variations to standard load-bearing and in-fill straw-bale construction systems. A hybrid approach uses elements of both, such as building three load-bearing walls and a fourth (on the south) that is framed to permit more glass. Texas architect Pliny Fisk has developed a hybrid system using vertical 1x4s on both the inside and outside of bale walls; bales are stacked in columns and sections of horizontal 1x4 span between the vertical 1x4s to form “ladder trusses.”
A very different approach is to mortar straw bales in place as is done with concrete blocks. This system was tested for structural performance in Canada in the 1980s. While it produces a quite rigid wall system, it uses a lot of high-embodied-energy Portland cement, and the thermal performance is not as good because of conductivity through the mortar joints. This approach is not discussed further here.
One of the primary benefits of straw-bale building systems is extremely high R-value. There is still little good data on thermal performance of straw-bale walls, and we found no actual energy consumption data for straw-bale houses. R-value measurements of straw bales were made in 1993 by Joseph McCabe as part of a Master’s thesis at the University of Arizona, and by R.U. Acton, P.E. of Sandia National Laboratories in May, 1994.
Results of these two studies are presented in Table 2.
If we average the results from these two sets of tests, we come up with values of about R-2.5 per inch (RSI/m-17.3) when bales are stacked flat (heat flow with the grain of the straw), and R-3 per inch (RSI/m-20.8) when bales are stacked on edge (heat flow against the grain). Note that testing to date has produced quite variable results, so these values can only be rough approximations. The bottom line, though, is that bales insulate extremely well.
Concern about fire is often the first reaction to the idea of building with straw. Aren’t straw-bale houses tinder boxes? Not at all, it turns out. While loose straw burns, once the straw is densely packed into bales it is remarkably fire resistant. The tight packing keeps the available oxygen needed for combustion very limited. Also, the high silica content in straw (3-14%) is said to impede fire—as it begins burning a layer of char develops, which insulates the inner straw.
As reported previously (EBN ), ASTM E-119 fire testing in New Mexico found that a plastered, 18-inch (450 mm) straw-bale wall survived fire penetration in excess of two hours, after which the flame source was discontinued; even an unplastered wall survived for 34 minutes.
The fire resistance of straw bales has been borne out by several fires in the past few years. In one house, a candle left burning in a niche that was carved into a straw bale wall started a fire. The fire completely burned the wooden surround and figurines in the niche, but the exposed unplastered straw in the niche did not sustain the fire; it was only charred.
In another situation, a wood-framed house wrapped with straw bales was under construction in Tucson, Arizona and had not yet been plastered. An arsonist started a fire, apparently with gasoline. The wood framing in the building burned almost completely, but the straw-bale walls did not. Unfortunately, because the bales were installed on edge, the strings burned and the building could not be salvaged.
Moisture problems and rot.
After fire, the second-most-commonly expressed concern about straw bale construction is moisture damage and rot. Straw is inherently resistant to rot, according to Ted Butchart. In The Last Straw, he claimed that few organisms are able to decompose straw, which is why “grain straw is so often burned rather than being turned back into the soil.” But high moisture levels in straw bales—over 70%—can provide a habitat for fungi and lead to decomposition, so careful design for moisture avoidance is critical.
The Fall 1994 issue of The Last Straw focused almost exclusively on straw-bale construction and moisture. Nearly all of the articles stressed the importance of keeping bulk moisture away from walls—for example, using wide overhangs, sloping the ground away from the building, and installing a good capillary break between the foundation and the bale walls.
Conventional frame construction often relies on a plastic film, vapor retarder paint, or other vapor diffusion retarder to help keep moisture out of walls. There is no clear consensus as to whether a vapor diffusion retarder should be provided on straw-bale walls. Most straw-bale building proponents recommend leaving walls relatively permeable to moisture diffusion on the premise that any moisture in the bales needs to be able to escape. Marc Rosenbaum, P.E., recommends climate-specific moisture detailing: in cold climates use a vapor retarder paint on the interior; in hot, humid climates use a vapor retarder paint on the exterior; in other climates the walls should probably be left permeable.
Donald P. Gatley, P.E. of Atlanta, Georgia, an ASHRAE Fellow, and co-author of several books on moisture, mold, and mildew, modeled a plastered straw-bale house in several different climates using the software program Moist to help Habitat for Humanity assess this building system. He concluded that plastered straw-bale buildings in the Midwest and Northeast should have vapor diffusion retarders on the interior. He did not think vapor diffusion retarders would be required in the Southwest, and they should not be used on the inside of walls in the Gulf Coast states (where air conditioning use is high).
Matts Myhrman, co-editor of The Last Straw, feels that we just don’t have enough experiential information yet on whether plaster walls should be permeable or impermeable. He noted that the early Nebraska bale houses never used a vapor retarder of any kind and moisture is not known to have been a problem, but he added that these houses often had outdoor privies and used wood stoves, which maintained quite dry conditions on the interior.
Insects and other pests.
Along with the concern about moisture and rot in straw-bale buildings is the concern about vermin. There are accounts of major problems with fleas in early hay-bale buildings in Nebraska that were not plastered on the interior. What about carpenter ants, termites, mice, squirrels, and other pests?
Myhrman says that almost all the termite species in North America (with one exception) do not eat straw; their diets are limited to wood. In an early straw-bale house in Wyoming, termites damaged window sills but left the straw intact. As for other pests, the key is to keep them out of the bale walls. “If you give rodents access to the bale walls, they will choose to live in them,” said Myhrman. But with careful plastering, you can easily keep out rodents and other pests. In areas with significant rodent problems, using a finer woven wire mesh than chicken wire for plastering might be a worthwhile precaution.
Structural testing of straw-bale walls has been very limited. Non-load-bearing wall assemblies constructed of two-string 18” (450 mm) straw bales were tested in 1993 by a certified testing laboratory in Albuquerque, NM as a requirement for obtaining building code acceptance. Transverse load tests were conducted to simulate wind loading. With 20 psf (98 kg/m2) loading, deflection was significant in an unplastered wall, but less than 0.13” (3 mm) with a plastered wall.
Load-bearing and non-load-bearing walls of three-string, 23” (580 mm) bales were tested at the University of Arizona in 1993 by graduate student Ghailene Bou-Ali. Individual bales were tested for compression, then nine wall panels 12 feet long by 8 feet high (3.6 m x 2.4 m) were built—though not plastered—and tested for compression, transverse lateral loading, and in-plane lateral loading.
Compression tests of individual bales found that bales laid flat can carry far more load than bales stacked on edge. Flat bales failed at an average load of 10,000 lb/ft2 (48,800 kg/m2); on edge, bales failed at an average of 2,770 lb/ft2 (13,500 kg/m2).
In the full-wall tests, compression testing was done with three assemblies that had not been pre-compressed. With loads of 1,317 lb per linear foot (1,959 kg/m), average deflection was 7” (178 mm). This loading is equivalent to a 82 lb per square foot of roof (400 kg/m2) if the wall carried half the load of a 30-foot (9 m) roof span with a 4:12 pitch.
In the other full-wall tests, walls were pre-compressed with threaded rod and did not carry top loads; again none of the walls were plastered. The transverse lateral load tests, which simulated a 100 mph (161 km/h) wind, resulted in less than 1” (25 mm) of deflection. The in-plane lateral loading resulted in up to 4” (100 mm) of deflection at the top of the wall when 2,135 lb (968 kg) was applied at the end of the wall. Complete test results are available for $15, including postage and handling from Out on Bale (un) Ltd.
Architect Chris Stafford of Seattle has had difficulty obtaining approvals for load-bearing straw-bale buildings in Washington, because he wants to use two-string bales (all that is available locally), and only wider, three-string bales have been tested in load-bearing wall assemblies. Thus, he has arranged for tests to be done during May by Washington State University.
Acceptance by building officials.
One goal of fire and structural tests on straw-bale wall assemblies is to gain acceptance by building officials. New Mexico is the first state to have granted approval of straw-bale construction, but only for non-load-bearing (in-fill) wall systems. The approval, which had been “experimental” (see EBN ), was formally announced in January 1995 as an “alternative materials permit.” It is available to anybody, according to Tony Perry of the Straw Bale Construction Association.
Load-bearing straw-bale buildings have been approved on a case-by-case basis around the country, usually under the “Alternative Materials and Methods” section of the relevant building codes, but the process is often difficult and drawn-out. A draft prescriptive standard for load-bearing and non-load-bearing straw-bale construction has been developed by David Eisenberg, of the Development Center for Appropriate Technology, with input from Matts Myhrman and others (see Table 3).
The standard is likely to be adopted into the City of Tucson and Pima County building codes shortly. Once that happens, other municipalities will have something to base their own building code amendments on, which should greatly spur the growth of straw-bale building.
For more information on getting a straw-bale building project permitted, refer to the working paper “Straw Bale Construction and the Building Codes” by David Eisenberg. This 28-page report is available for $11 postpaid from Out on Bale (un)Ltd.
Manufactured Panel Products from Straw
While straw-bale construction systems have gained most of the media attention in the past few years, this is not the only area of activity with straw. There are at least ten companies either currently building or planning to build manufacturing plants in North America to produce compressed-straw building panels with applications ranging from interior partitions to particleboard (see Table 4).
Several companies expect to build multiple plants. If all of the plans currently underway reach fruition, there could be several dozen compressed-straw-panel manufacturing plants in North America within five years.
Compressed-straw panels are not new. The process for producing “Compressed Agricultural Fiber” (CAF) panels was invented in Sweden in 1935 by Theodor Dieden, then developed into a commercial product in Britain under the name Stramit by Torsten Mossesson in the late 1940s.
Since original patents have expired on the technology for producing CAF panels, numerous companies using the Stramit process have sprung up worldwide. Stramit manufacturers are going strong in several European countries and Australia, and Stramit Industries, Ltd. of the U.K. claims that over 250,000 buildings have been built using these panels.
All of the products that use the basic Stramit technology make use of an interesting property of straw: when straw is compressed under high temperature (about 390°F or 200°C), the straw fibers bond together without any adhesives. Some manufacturers claim that the resins in the straw somehow fuse together, but Robert Glassco of Pyramod International, Inc., one of the pioneers in the United States, claims that the straw doesn’t really fuse together.
Instead, he says, heat makes the straw become limp and form around each other from the compression. When the material is cooled, the fibers stay in place. “It’s really a sophisticated bale,” he claims. Samples EBN has examined seem to hold together very well.
Stramit panels range in thickness from 2” to 4” (50 to 100 mm) and are faced with heavy-weight kraft paper (similar to that used in drywall). Though adhesive is not required to bond the fibers together, it is required to secure the facings. These Stramit panels are used primarily for interior applications where they can provide complete partition systems. Most of the products are pre-routed for electrical wiring, and clips are sold to join panels securely together.
A number of companies are pursuing the idea of gluing several Stramit-type panels together, adding protective facings, and using the panels as structural insulated panels that can be used as the exterior envelope of buildings. In an attempt to restart production, Pyramod Industries has erected several prototype houses (see photo on page 16) using panels that have been stockpiled since their factory shut down in the 1980s.
Because baled straw is a low-density material, shipping costs are high—both in dollars and environmental impact (primarily from fuel consumption). Will Maertens, an architect and principal of AltMatTec told EBN that shipping distances are a major determinant of the economic viability of manufacturing panel products, no matter what the raw material is. Wood pulp, with a density of 15-20 lb/ft3 (240-320 kg/m3), can be cost-effectively shipped up to about 40 miles to a manufacturing plant, he said. Straw, which has a density of about 8.4 lb/ft3 (134 kg/m3), can only be shipped 18-20 miles (29-32 km) before shipping costs become a major economic obstacle.
The thermal performance of compressed-straw panels is a matter of much confusion. Claims range from R-1.25 to R-4 per inch (RSI/m-8.7 to 28) for products that appear essentially identical and have densities in the range of 15 - 23 lb/ft3 (240 - 370 kg/m3). After investigating many of these claims, EBN believes a realistic range to be R-1.4 to R-2 per inch (RSI/m-9.7 to 13.9). Some of the highest claims are probably simple misconversions from the original metric units, though one company, Agriboard Industries, provided a summary page of test reports from a certified testing laboratory in Minnesota to support their claim of R-3.4 per inch (RSI/m-23.6). When EBN contacted the laboratory, a technician indicated that the test method quoted had not been used there for many years, and he questioned the high R-value given the material’s density. This issue is extremely important in terms of how appropriate compressed-straw panels are for building envelopes; EBN hopes to see up-to-date independent test results from manufacturers.
Thin Panel Products.
Also on the way are higher density straw particleboard panels. Two companies have plans to produce furniture-grade particleboard out of finely chopped wheat straw. Both will be using an isocyanate binder (MDI), which is more weather resistant, non-offgassing, and stronger than the urea-formaldehyde usually used for particleboard. The companies claim that their products will outperform conventional particleboard.
Thin panels are also made out of longer straw fibers. Meadowood of Albany, Oregon had a fiberboard product on the market for several years made out of rye grass straw, but the company has ceased production temporarily. Sea Star Trading Company, a dealer in ecological timber products, will soon reintroduce the product with an optional hardwood veneer.
Conclusions & Predictions
There is little doubt that straw could become a significant player in the building industry. Tremendous quantities of straw are produced in North America—over 140 million tons (128 million tonnes) annually , based on EBN's calculations. In grain-producing areas it is usually available at very low cost. Just how much of that straw could be used without negatively affecting soils is unknown—and an important area of research. If we were to assume that 25% could be used without harming soils, we would have a resource base of about 35 million tons (32 million tonnes) per year. Let's take a look at what this quantity of straw could mean for the building industry.
If we used all of that available straw for the exterior walls of straw-bale buildings, 2.7 million 1,000-ft2 (93 m2), single-story houses could be built each year. If we turned that straw into structural compressed-straw panels, they could provide the exterior walls, roofs, interior partition walls, and floors of 1 million 2,000 ft2 (186 m2), two-story houses per year. Or, that straw could be used to produce 22 billion ft2 (2.1 billion m2) of 3⁄4" (19 mm) particleboard, which is five times the current total U.S. production of particleboard and medium-density fiberboard (all thicknesses). Clearly, the potential is significant.
Whether straw-bale building systems can ever catch on widely is unknown. It is so different from typical building approaches today that straw-bale building has been limited primarily to owner-builders. But the Straw Bale Construction Association, a fledgling trade association of straw-bale builders and architects, is growing and has members in 22 states, so there is obviously some level of interest among professionals. Because of shipping costs, straw bale construction will very likely remain limited to areas with local sources of baled straw.
Panelized building products produced from straw will not be as limited either geographically or in terms of acceptance by the building trades. For these reasons, EBN looks to these products for the most rapid growth and acceptance in North America. Given the number of manufacturers getting involved, we expect to see a great deal of activity in this area in the next few years. If the building industry adopts these products there will likely be some large players getting involved, which could dramatically hasten the growth.
Straw-based particleboard products (PrimeBoard and Isobord) will likely lead the pack, because these are drop-in replacements for conventional wood-based products (with superior performance no less). Of the Stramit-type products, the biggest cloud threatening dramatic growth is thermal performance. If the R-values prove to be much less than R-2/inch (RSI/m-13.9), they will not be ideal for exterior envelope applications. Exaggerated or fraudulent performance claims could negatively affect the entire industry. For interior partition walls, the primary applications currently being pursued, low R-values will not make much difference.
We at EBN are excited about the prospect of straw and other waste agricultural fibers becoming widely used in the building trades. As with many new ideas, the use of straw will meet lots of resistance along the way, and it may find very slow acceptance, but we feel that its environmental and economic benefits will ultimately result in its widespread use. We will report on new straw-based materials as they are introduced and keep readers apprised of developments in straw-bale building.
For More Information
Out on Bale (un) Ltd.
1037 E. Linden St.
Tucson, AZ 85719
Publisher of The Last Straw, the leading newsletter on straw-bale construction, and a clearing-house for information on this building system. Matts Myhrman and Judy Knox.
Development Center for Appropriate Technology
P.O. Box 41144
Tucson, AZ 85717
e-mail: strawnet@aol. com.
Organization developing prescriptive standards for straw-bale construction. David Eisenberg, president.
Straw Bale Construction Association
31 Old Arroyo Chamiso
Santa Fe, NM 87505
A new trade association comprised of straw-bale builders, architects, engineers, and contractors. $40/year. Tony Perry, president.
1509 Queen Anne Avenue North, #606
Seattle, WA 98109
206/284-7470 (phone & fax)
Ted Butchart and Peggy Robinson, co-directors.
There is also an Internet listserv on straw bale construction. To subscribe, send the message "subscribe strawbale" to email@example.com.