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CASBA gratefully acknowledges the following contributors for their help in creating these FAQs:
An additional online resouce for SB construction is the Straw Bale Construction Wikibook maintained by Duncan Lithgow.
Questions about building with straw bales
Plowed into the ground, most straw takes six months to decompose. Rice straw, which has a high silica content, takes twice that time. Straw has been used as an insulating material for many centuries, and has been found in excellent condition in Egyptian tombs thousands of years old. If kept dry, straw will not degrade. It can be said, then, that the lifetime of straw in a building could be anywhere from three weeks to nine-thousand years, depending on how well the building is constructed and cared for.
Compared to wood, there are few termites who like straw. At least once, termites entered a building, left the straw alone, and ate the wood windows. The normal precautions against termite infestation used in wood construction should be followed. Bales provide fewer spaces for pests than conventional wood framing, where, should rodents enter a wall at a break in the plaster coating, they would be likely to make a place to stay. It would be very difficult for pests to travel though the bales, however. Unlike hay, straw contains very little nutritional substance and will not, in itself, support a pest population. Conventional precautions against pests should be more than adequate for straw bale. Some concerns have been raised about hay fever, toxins and pesticide residues. Of course, once the bales are encased in plaster it is difficult for irritants to transfer into the dwelling. Clean, dry bales contain few molds or pests. And pesticides, at least for rice straw, are used early in the growth cycle, then discontinued. Rice straw contains extremely small amounts of pesticide residue.
Test results show straw bale construction to be exceptionally resistant to fire. A test of a plastered wall panel showed a two-hour fire resistance, and an unplastered bale wall had a 30-minute resistance. Unlike stud construction, in which a series of chimneys (stud cavities) form the wall, bales are dense and difficult to burn. And, since plaster applied to the uneven bale surfaces tends to be thicker than normally found on buildings, the bales can be said to carry an extra layer of protection. Loose straw, which is sometimes used to fill cavities, is much more vulnerable to fire and should be dealt with carefully by sealing with plaster or treating with fire retardants.
Fungus (dry rot) can occur in straw at sustained high levels of moisture (over 20 per-cent of dry weight, or relative humidity of 70 to 80 percent)-- significant damage occurs when these levels are maintained over a long period of time. Intermittent moisture is not a threat, however. In California, we can face periods of intense rain, usually followed by dry, windy days, excellent for drying out walls. It is rare, for instance, to see moss growing on exterior plaster, as one would find everywhere in England. Should moisture on the outside face of a bale wall rise above 20 percent during an extended rainstorm, it would nevertheless dry out before extensive fungal growth could occur, if allowed to breathe.
Experience and test results suggest that the best way to avoid sustained high moisture concentrations lies in making certain that the bales are able to transpire any accumulated moisture back into the environment. Building paper, commonly used to cover plywood walls, could inhibit the bales' ability to transpire moisture to the outside, and create a surface where moisture could concentrate for extended periods. Many experienced straw- bale builders recommend a breathable sealer to prevent water from penetrating the stucco from the outside but allow moisture vapor to transpire through the stucco to the outside. Historical data, for bale walls without moisture barriers, suggest the importance of walls of maximum breathability: a mansion in Huntsville, Alabama has successfully endured Southern humidity since 1938; a 1978 building near Rockport, Washington receives up to 75 inches of rain a year; and a building near Tonasket, Washington, with no foundation and unplastered walls, shows no apparent deterioration of the bales since 1984. Recent bale structures in northern New York (humid winters) and Nova Scotia (cold humid winters) have been monitored and demonstrate good performance in these difficult climates. Because of the large amount of moisture that occurs at the bottom of a wall due to ground splash a vapor-permeable covering such as Tyvek outside the lower courses of the walls is recommended. The top and bottom of a bale wall am also vulnerable to moisture. Consequently, experienced straw-bale builders recommend building paper over the top of bale walls, and a capillary break, such as gravel (rather than a waterproof membrane), underneath the bales.
The surfaces of straw bales offer an excellent mechanical bond to plaster and stucco, and reinforcement is generally not needed to attach plaster to the walls. Reinforcement may be desired when stucco is used as part of the structural system, or as assurance against hairline cracking. When needed, a variety of techniques can be used to attach netting, including long staples stuck into the bales or wire ties through the bale walls. Because of the natural undulations of a bale wall, an irregular pattern of attachment, rather than a simple grid, works best; care must be taken that the netting is uniformly secure. Also, wherever the bales abut a dissimilar material (e.g., at the mudsill, the ceiling and abutting stud walls), the wall should be reinforced with expanded metal lath that extends at least six inches onto the face of the bale.
While the California Health and Safety Code sanctions both load-bearing and non- load-bearing systems, most California builders use a wood post-and-beam system that carries vertical loads in a conventional manner. Wind and earthquake loads are carried by means such as diagonal steel straps, which can be conventionally engineered. The bale walls thus are primarily subjected to wind and earthquake loading against their faces. Test results show that plastered wall perform well with wind loads of up to 50 pounds per square foot. However, the bale walls add a significant secondary structural system Compared to wood- framed structures, bale buildings are resilient and flexible. We believe the bale walls can absorb some of the force of an earthquake and will provide a backup structural system in the event of failure of the post-and-beam system.
Contrary to popular misconception, the interior of a building-quality bale is not loose and fluffy, as the exterior often appears. The straw on the bale interior is quite dense and holds stakes or dowels driven into it very securely. When fastening items on the surface of a wall, we commonly cut 12-inch-long tapered stakes from a 2x4, drive them into the wall, and secure items to these. This is more than adequate for securing electrical boxes and typical shelving. For extra-heavy loads, we run bolts through the wall to oversized washers on the opposite side. When securing window and door jambs in openings, we can take advantage of the even greater resistance encountered when driving elements perpendicular to the 'grain' of the straws. in these instances, we use stout wooden dowels as oversized 'nails' driven through pre-drilled holes in the wood bucks.
Many builders use the precaution of installing pipes, which could sweat or leak, inside continuous sleeves within bale walls.
While ordinary non-metallic cable, such as Romex®* brand cable, is often used in bale walls, UF cable (rated for direct burial)
can be used where extra caution is desired. The Romex® brand cable, other nonmetallic cable, or UF cable is set three inches into the bale walls, safe from punctures. This also sets it into the firm portion of the bales, where it can be securely pinned. Electrical boxes are typically screwed to tapered stakes driven flush with back of recesses cut into the straw.
* Romex® is a registered trademark of Southwire Company.