BY DON FISHER
Straw and clay have been used as building materials for more than a thousand years. Many medieval European structures are still standing today. The popularity of straw-built construction, based on the use of straw bales, can be traced back 140 years to Nebraska and the invention of the mechanical baler. The designers, builders, and proponents of this type of construction report its many advantages. Its insulation, acoustics, and ambiance properties and the fact that it is a green, or earth-friendly, material are just a few.
This type of construction has grown since the meeting of the modern pioneers of this construction in Oracle, Arizona, in 1989. The greatest increase has been in the southwestern United States and California. Throughout the United States, this construction has been used to build high-end luxury homes, post offices, monasteries, stores, schools, police stations, and ordinary houses. It has been used also for health clinics in China, wineries in Australia, palaces in Saudi Arabia, and thousands of styles of houses throughout the world. I am not promoting or condemning this type of construction. My intent is to make firefighters more aware of this construction and the challenges they will encounter when fighting fires in these structures.
THE INCIDENT
On December 10, 2007, the Fort Mojave Mesa (AZ) Fire District (FMMFD) responded to a call dispatched at 12:27 p.m. for a fire in a straw-built residential structure. The initial response consisted of two engines, a rescue, and a battalion chief, with an additional engine coming from mutual aid. The wind was out of the North (D side) at 30-35 miles per hour (mph) with gusts to 45 mph. The structure was a 5,000-square-foot single-family residence of straw bale construction, stucco exterior, and dry wall interior finish with straw bales used for insulation and structural support, red steel metal frame, and a concrete tile roof. In this particular house, the straw bales do not provide structural strength. They only support themselves, provide insulation, and support the interior and exterior finish material.
Initial Actions
The initial dispatch stated that this was a straw bale building. The crew that was on duty that day had seen this dwelling being built and had responded to a fire in the structure years earlier, prompting FMMFD Battalion 1 (B1) to immediately request an additional mutual-aid engine, Water Tender 92, and the deputy chief (safety officer).
 (1) View from the north B side on firefighters’ arrival. [Photos by Darrel Rayburn, Fire Marshal, Fort Mojave Mesa (AZ) Fire District.] |
Engine 92 arrived on-scene within six minutes of the alarm and reported smoke showing from the A/D corner of the roof area. The owner met E92 and said that he was soldering in that area and that there was fire in the wall under the sink. He also advised that the structure was of straw bale construction.
B1 arrived, assumed command, and set up a command post east of the A side of the building. B1 ordered a 1¾-inch attack line to the B side, T92 to set up an initial static water supply (3,500 gallons), and E5 to lay a five-inch supply line to a hydrant that was 500 feet away and established a rapid intervention team.
On my arrival, I was appointed safety officer. The captain from E92 informed me that the house was straw bale construction and that there was fire in the wall. On my first of many ongoing complete size-ups of the exterior of the building, I reported to command that the fire was already in the attic.
 (2) Thermal camera image of the wall where the fire started. |
E92 used the thermal imaging camera (TIC) to determine the location of the fire in the wall and started opening the wall from the outside with hand tools. This was a difficult and time-consuming task, since, as is the case for most houses in the Southwest, the exterior of the building was stucco, which is cement-like material that is plastered to wire mesh (chicken wire with stucco plastered to wire mesh backed with tar paper). This type of exterior finish makes it very difficult, in comparison with other building materials such as dry wall or plaster lath, to remove or breach. E92 then got a rotary saw with a metal blade and sledgehammers to open up the outside walls.
The TIC indicated fire in the wall from under the sink to the top of the wall. The compounding effects of high winds, a delayed call, and a lack of firestopping with combustible insulation material caused the fire to rapidly extend into the attic.
 (3) Outside view of the point of fire origin. |
After the wall was opened, the intensity and volume of the smoke increased as a result of the wind’s positive-pressure effects. The E92 crew used this natural ventilation to its advantage by opening doors and windows. This was effective on the interior, as smoke conditions below the attic were relatively clear.
Planning
After being on-scene for 10 minutes, I, Safety, met with Command. We discussed that the house was straw-built construction with a cement tile roof, backed with oriented strand board (OSB), and was being supported by lightweight steel frame trusses. Because there was heavy fire in the truss void, the decision was made to withdraw the two interior attack crews and take a defensive position, because there were signs of collapse. We conducted a personnel accountability report. Within minutes, the metal trusses began to twist, causing portions of the roof to fall into the kitchen area (area of origin). Command called for a second alarm. The second alarm brought in an additional engine, a call for additional staffing, an air unit and a mechanic, and a fire investigator.
The strategy at this point was to pull ceilings with 12-foot pike poles through the windows from the exterior D-side, under the back porch roof that did not have fire in it. This provided a safe zone from the tiles falling off the roof and allowed crews to gain some access to the attic void space to get water into the attic and to conduct a recon with the TIC to determine what part of the house could be saved.
 (4) The roof collapsed 10 minutes after the fire department arrived on the scene. |
The results of the recon with the TIC indicated that fire was in approximately 75 percent of the attic and that fire was traveling downward through the straw-filled walls in about 50 percent of the house. It was later determined that the top of the walls had no drywall on the top plate and that there was a space between the straw and the inside and outside wall. This flaw in construction contributed to the total loss of the building.
Because of the high winds and the exposed OSB roof decking, the 5,000-square-foot building’s attic was almost totally involved. This caused the top of the unprotected straw bales to ignite from embers and burning materials that fell into the void between the bales and the interior and exterior walls. You could see fire with the TIC on the outside walls on the top of the bales and on the bottom of the bales; there was no fire in the middle of the wall.
Sustained Actions
As a result of the recon that showed we had smoldering fires burning in about 75 percent of the structure’s walls, a plan was devised to try and save the remaining part of the house. The plan included the following:
- An interior attack on the portion of the building that had fire only in the attic and not the walls.
- Using the owner’s backhoe tractor to strip the exterior walls in the portion with fire in the walls.
- A call for a compressed air foam engine (CAFS) and additional personnel.
- A salvage operation to retrieve valuable personal articles and Christmas presents from the home.
The interior attack on the savable C end of the building was successful. The RIT was repositioned, and the attack crew entered, pulled ceilings, got water into the attic, and was able to stop the fire.
 (5) The dark spots represent fire in the wall as seen through the stucco exterior finish. |
The knockdown at this end of the building prevented the fire from spreading downward into the walls. At the same time, the exterior crews were working with the backhoe operator to tear down the exterior walls. This exposed the burning straw; the fire was extinguished with class A foam and CAFS.
TERMINATION
Firefighting operations began at 1327 hours, and a “Loss Stopped” was called at 1825 hours. Mutual-aid companies were released. A fire watch crew of four remained on scene until 2059 hours.
 (6) The part of the home saved by the interior attack. |
A lighting unit, contracted from a local company, was used to light the scene for crews assigned to the fire watch; straw bale fires smolder for hours. B1 returned to the scene at 2330 hours and was satisfied that the fire would not rekindle.
At the end of the day, the fire consumed 75 percent of the building. The exterior walls were mechanically breached during overhaul operations so firefighters could gain access to the smoldering straw bales, making the building a total loss. The insurance company later estimated the loss at nearly one million dollars.
THE INVESTIGATION
The investigation revealed that several design flaws had contributed to the fire spread. The use of experimental products is allowed under the building code if the products have been tested and approved [International Building Code (IBC) Section 104.11]. The problem is that straw, like the blown treated paper insulation, is combustible. Both will burn when temperatures reach 1,200°F. To combat this, the straw stacks were designed to be treated with fire retardant and covered airtight with an exterior covering. In this case, a modification was made for aesthetic reasons.
 (7) The steel frame was exposed during overhaul; the straw bales were opened and extinguished. |
Furring strips were added to allow wallboard to be placed on the interior, for a smoother look. This created an air space that eventually created a chimney effect. There are no codes to prevent this from happening.
The owner said he was experiencing cold drafts around the various wall openings and attempted to have additional insulation blown into the space at six-inch intervals. The codes do not address this issue either.
Because the design of straw construction is to create a solid space, the walls are stacked on each other with no fire blocking in the walls. Current codes require all wooden constructed walls to have fire blocking (IBC 717.2). It was pointed out later that the top of the wall is the only fire blocking in straw houses. It is designed to have a top cap placed over the wall unit. This isolates the straw and limits fire spread. Without it, fire can extend up to the attic, as did this fire.
A second issue was observed with the roof. The roof was designed with OSB laid over a steel frame. Cement tiles were then attached to the top of the OSB. There were no code requirements for protecting the OSB. In this fire, the roof was compromised early, and the fire continued to spread readily because of a lack of fire protection.
The third issue was the fact that with “green building” construction, the codes strayed away from conduit and raceway requirements. Straw houses are not designed with conduits or raceways. The electrical wiring was laid in the wall and had a protective covering. The plumbing was cut through the wall and was not placed in a raceway, as is commonly required in concealed spaces.
The fourth issue is the human factor of not following safe and proper procedures while doing hot work. Hot work is covered in Chapter 26 of the International Fire Code. This fire spread in a worm-like fashion. The TIC would detect glowing hot embers, and it would be cool six inches away.
Those who responded to this fire will never forget the five-hour fight to extinguish the fire. Most may never fight a fire like this again. In my 30 years as a firefighter, I had never responded to a fire in a straw-built structure, nor have I seen a homeowner tear down the walls of his own home. I hope I will never see it again.
LESSONS LEARNED
- Fire crews should tour construction areas to see how buildings in their response area are constructed.
- Departments that do not have truck companies available must be able to perform typical truck company functions and have the necessary tools to do so.
- Consider using piercing nozzles.
- Consider using Class A foam early.
- It is imperative to use a TIC when fighting fires in concealed spaces. All crews could use one.
- If you encounter a fire in straw bale construction, call for help early.
- Consider using mechanical methods for breaching.
- All buildings, regardless of construction type, must comply with all applicable codes.
- When you think you have seen it all, just go to work another day.
References
Bolles, Bob. Straw Bale Construction 101, 2007.
Coppinger, Joyce, managing editor, http://www.thelaststraw.org/.
King, Bruce. Design of Straw Bale Buildings. Green Press. 2006.
http://www.ecobuildingnetwork.org.
The Last Straw Journal, GPFS/TLS, P.O. Box 22706, Lincoln, NE 68542.
Straw Building Construction 101
BY BOB BOLLES
Straw is the annually renewable agricultural by-product of cereal grain, which is often burned as waste. The grains include rice, wheat, oats, and barley. Most of the available straw in the western United States is from wheat, where a great deal of wheat is grown along the Colorado River. Baling machines, or balers, are used to convert the loose straw into bales. The bales generally are of two sizes. Two-string bales are approximately 14 inches high, 18 inches wide, and 36 inches long. They weigh 40 to 50 pounds and are found mainly in the northern and eastern states. Three-string bales are approximately 16 inches high, 24 inches wide, and 48 inches long. They weigh 70 to 80 pounds and are found predominantly in the southern and western states.
TYPES OF BALE BUILDINGS
The first bale buildings in the United States were constructed in the Sand Hills region of Nebraska. They were load-bearing (sometimes referred to as Nebraska style), which means that the bales supported the weight of the roof. Most of the straw bale construction (SBC) structures built today (especially in areas subject to seismic and wind loads) are nonload-bearing. A structural frame of wood or steel is used to support the roof weight and lateral-load bracing needed to resist the extra seismic and wind loads. The majority of construction elements and code requirements for SBC are the same as for conventional construction.
Bale building foundations are normally the same as standard slab-on-grade construction systems. The bales rest on sills (typically a pressure-treated bottom plate) beneath the outer edge of the bales. The material between the sills is typically graveled, creating a capillary break; however, some designers and builders use a rigid insulation to create a thermal break as well. Typically, the outer plate is bolted to the foundation, as per code requirements. The inner plate is attached with power-driven concrete nails. Simpson brackets are used to attach vertical structural members to the plate/foundation.
In wood-framed construction, posts (4 × 4 and 4 × 6) extend from the plate/foundation to a wood beam (sized by the engineer to support the type of roof load (i.e., 4 × 12), joined by the appropriate connector. Although there are a variety of structural systems used to provide resistance to lateral loads, the most common assembly is a Hardy Frame, which consists of a heavy-gauge sheet steel channel with a diagonal brace of the same material. These assemblies are connected to the foundation with bolts and to the bottom of the beam with screws.
Windows and doors are mounted in a variety of framing systems. I use a 4 × 4 for the post and a rectangular frame perpendicular to the wall face on each side of the opening. Because these panels are placed at the ends of the bale wall and not at the inner and outer wall surfaces, we attach OSB or exterior-grade plywood to the outer face of the frame to provide a flat surface to which we attach the windows and doors. The roof framing system is typically conventional.
BALES
Three-string bales are the majority of bales available in the western United States. Most of the bales that we have been using in the past few years are closer to 23 inches wide, 45 inches long, and 15 inches high. Most bales are used in the “flat” orientation (15 inches high), with the strings oriented on the top and bottom. Six bales are stacked to create an eight-foot-high wall.
The bales are laid in a “running bond,” where they are half-lapped over the bales below (like bricks and concrete blocks), increasing the stability. Because the bales are of unequal length, they do not create a perfect half-lapped wall; some bales are shorter and some are longer. To fill in the width of the shorter bales, loose flakes (no wider than six inches) are typically stuffed between the short bales to keep the lapped ends centered over the bale below.
There are a number of different ways to frame up a post-and-beam wall. Perhaps the most common system is to place posts at the corners, the (engineered) designated spacing under the beams, and on each side of a door or window opening. The framing for the opening is attached to these posts.
Where a series of bales are too short to reach the end of a row-section, a partial bale is made to fit into the opening. Bales are not cut into shorter lengths; instead, a bale needle (a steel rod with a notch in the end, in which a piece of twine is inserted) is pushed through the side of the bale at each existing string location, wrapped around the desired partial bale section, and tied tightly. After all three strings have been secured, the original strings are cut, and the excess is removed. (Sometimes, two partial bale segments are made at one time.)
Prior to bale stacking, a half-sheet of 5⁄8-inch Type-X (cut lengthwise) drywall is attached to the ceiling joists with the cut edge butted against the top edge of the beam; the joint between the beam and the drywall is caulked to make it airtight. (The drywall extends out past the inner wall edge the width of the beam, allowing adequate space to tape and finish the joint when the remainder of the drywall is installed after the framing inspection.)
The first row of bales is stacked on top of, and carefully aligned with, the bottom plates. From the top, loose straw is tightly packed into any voids between the bales.
Subsequent rows are stacked, aligned, and half-lapped with the bales below. Typically, the top row of bales is notched into the beam. When the bale walls are completed, a mixture of clay and straw (referred to as cob, a material that has been used in European home construction) is used to fill all voids.
LATH
In California, seismic load testing has been conducted on load bearing-wall sections. In these tests, it was determined that 16-gauge 2-inch × 2-inch welded wire mesh (WWM) attached (wired together through the bales) to both sides of a straw bale wall was adequate to meet the structural requirements of a load-bearing wall in a seismic event. We have adopted this system (using 14-gauge 2-inch × 4-inch WWM) in our nonload-bearing structures, to function both as a plaster lath and to resist the seismic out-of-plane movement of the bales within the engineered structural frame of a post-and-beam SBC wall.
Where wind and seismic loading are not structural issues, standard 18-gauge wire mesh is used for plaster lath. Under no circumstances should a vapor barrier (such as builder’s paper, plastic, or Tyvek®) be used in plastering a bale wall. Very often, diamond (expanded metal) lath is used to reinforce the corners of window and door openings, corners, and rounded edges of openings.
PLASTER
Plaster protects the bales from moisture, insects, rodents, and fire. The plaster must be bonded directly onto the bale walls without any vapor barrier or sheet material between the two. The joint between the plaster and any adjoining materials must be caulked to eliminate air leakage.
A variety of plaster materials are used over bale walls. The most common plaster is cement plaster (stucco), which consists of a binder of portland cement (not “plastic” cement), lime, and sand. Lime plaster is highly vapor permeable but necessitates more time and care for the curing process. Clay plasters (often referred to as adobe plaster) are becoming increasingly popular with straw bale owners.
Clay has become an integral part of most SBC, used in several other types of applications. The most common use is to mix it as “cob” or “light clay.”
RAIN-SCREEN CLADDING
In very windy or wet locations (wind-blown rain) where plaster is undesirable for architectural (aesthetic) reasons, siding can be used to cover bale walls. Vertical sidinge.g., board-and-batrequires horizontal furring strips, and horizontal siding requires vertical furring strips, or furring strips are attached to, or through, the bales as attachment points for the siding.
ELECTRICAL WIRING
Some municipalities have required that wiring in bale buildings be encased in conduit, but I have not encountered this stipulation in San Diego, Riverside, San Bernardino, or Imperial Counties of Southern California. In my experience, electrical fires occur in outlet and switch boxes and are caused by overloading of the circuit and/or bad connections. All of our wiring has been Romex®. You can string wires in bale walls in one of three ways:
1. Cut a slot about three inches deep into the bales with a chainsaw (or other cutting device). Temporarily hold the wire in the slot with U-shaped wires and then use a cob mix to hold them in place.
2. Place the wires on top of the first course of bales and then extend them to the wall surface at each box location.
3. Set the bottom plate back from the edge of the bales (about two inches), providing adequate space for the wires in the void created. Place the wires in a slot from the void to the box location. Cover the void/wires with base molding along the bottom. To hold the boxes in place, drive wedge-shaped 12-inch-long 2 × 4s into the bale wall and screw the box to the flat end of the wedge.
Other systems are used as well.
PLUMBING
It is strongly recommended that no plumbing be run through bale walls. As an alternative, where plumbing has to be next to a bale wall, run it through the floor. Vent pipes are the exception, but an alternative location would be preferable if possible.
BOB BOLLES is a straw built construction (SBC) contractor in Poway, California. He became an advocate of SBC in 1995. His company, Sustainable Building Systems, provides consulting services to homebuilders. His teams have designed more than 60 homes and have assisted in building 40 additional SBC buildings. He conducts workshops and instructs in SBC.
DON FISHER has been a member of the fire service since 1977. He began his career as a firefighter in the United States Air Force. He joined the Ann Arbor (MI) Fire Department in 1981 and rose through the ranks to become chief of training; he retired from that position in 2002. He then became the chief of training for the Fort Mojave Mesa (AZ) Fire District, where he served as deputy chief after five years. He has been an instructor for Michigan State University for 20 years and is the fire science coordinator for Northland Pioneer College in Northeast Arizona. He has an associate degree in fire science and a bachelor’s degree in interdisciplinary technology and is pursuing a master’s degree in fire department administration at Arizona State University.
