Straw bale construction is a green building method that uses whole or partial bales of straw as a major part of a home's walls. Some builders construct frames around the bales and use the bales for insulation, while others eliminate the framing altogether. Straw bales are inexpensive and, as long as they haven't been chemically-treated, are an all-natural building material. Because they are so well insulated, straw bale homes are easy to heat in the winter, and they stay cooler in the summer. On the downside, straw bale construction presents a host of special challenges, from getting the project designed and built to maintaining the building over time.
Straw bales can be heavy and bulky to move, especially when building the upper courses of a straw bale structure. Many straw bale homeowners opt to help with the construction, in part to reduce labor costs. Even contractor-built homes can suffer from the increased labor requirements of straw bale building. Despite their cheaper materials, straw bale homes cost 20 percent more than conventional structures when built by a contractor, according to StrawBale.com.
Wet conditions can cause straw bales to mold, decay and even collapse. Properly-constructed straw bale homes include a high stem wall, roof with large overhanging eaves, and interior and exterior plaster coats to reduce moisture problems. However, these precautions sometimes fail. In wet climates with blowing rain, straw bale structures may become damp and may never dry out. In this kind of situation, it may be best to build only the driest sides of a structure from straw bales, and use another construction method for the wetter sides of the house.
Unlike most modern homes, straw bale structures require regular maintenance to remain habitable. All straw bale houses need a layer of plaster on the outside to keep water out. While cement stucco is convenient and low-maintenance, it can cause moisture problems once it cracks, and may actually keep the bale wall from drying properly. Most straw bale builders use clay-based earthen plasters or lime plaster, which require regular reapplication. Gypsum plaster works only on interior walls, as it dissolves in the rain.
Straw bale homes are especially sensitive to movement from under or within the structure. According to The International Association of Certified Home Inspectors incorrectly built straw bale homes may suffer from structural movement, which cracks the plaster and could cause eventual collapse in load-bearing walls. This structural movement can come from under the foundation, due to overly wet, poorly compacted or expansive soil. An improperly designed foundation or seismic activity can also move the home. High winds, especially when combined with an inadequate structural frame, can also cause cracking and damage.
Straw bale construction remains relatively uncommon in the building world. Most contractors and home inspectors are unfamiliar with straw bale building, which can make it difficult to get a professionally built, properly approved structure. Banks are often unwilling to finance straw bale homes, which have unpredictable resale values, and insurance agencies may not be willing to write fire insurance policies for these homes.