Straw Bale House Inspection
Straw-bale construction is a building method that uses bales of straw as structural elements and/or building insulation.
Facts and Figures
- The first documented use of hay bales in construction was a Nebraska schoolhouse built in 1896 or 1897. Unfenced and unprotected by stucco or plaster, it was reported in 1902 to have been eaten by cows. From then on, builders began plastering their hay-bale structures.
- Small sections of the interior wall left unplastered, known as “truth windows,” are sometimes incorporated into straw buildings. The opening may be glazed or framed.
- Straw bale is used in two basic styles of construction: “Nebraska”-style structures, which resemble Southwestern adobe-style architecture and use the straw for structural support; and post-and-beam construction, with straw-bale infill for insulation. Some structures incorporate both of these styles.
- Common types of straw used in construction are wheat, rice, barley, rye and oats.
While the construction of modern straw homes has seen a recent resurgence, straw has been used in construction on the African plains since the Paleolithic Era. Straw-thatched roofs were common in northern Asia and Europe in past centuries (and many original thatched roofs are still maintained today, especially in the U.K.), and straw homes were built in Germany 400 years ago. American Indian teepees were typically insulated for winter with loose straw placed between the outer cover and inner lining. Early settlers of the American West used straw as thatch roofing, but it wasn’t until the 1890s that the mechanical baler allowed straw to be used as construction building blocks. Their popularity waned by 1940 but re-emerged in the 1970s. Straw bale homes were finally recognized by some building codes by the 1990s.
- makes use of a resource that would otherwise go to waste. Each year, approximately 200 million tons of straw are burned in the United States;
- makes use of a local resource. Straw can usually be found nearby, which significantly reduces transportation costs ordinarily required by lumber and metal;
- tends to go rapidly because the building blocks are simple and large, and require less expertise than conventional building practices;
- is relatively inexpensive because of the low cost of the raw material, which otherwise tends to get burned up by farmers due to its limited use;
- makes a home more earthquake- and wind-resistant. Straw is ductile and will bend to accommodate an earthquake. Straw bales are thick enough that they will resist strong winds and high-speed, windborne debris;
- makes a home fire-resistant. While loose straw is flammable, tightly packed bales that are properly constructed and plastered actually perform better against fire than most conventional insulation. A straw bale withstood temperatures of 1,850º F for two hours in a test conducted by the National Research Council of Canada;
- makes it energy-efficient. Due to its high R-value, heating and cooling expenses are generally less in straw homes;
- makes it soundproof. Noise from nearby highway traffic, an overhead flight path, or rowdy neighbors will largely be kept out of a straw home. Even some recording studios use straw as a noise-mitigating building material. Noise control is also influenced by a number of other factors, such as the type of doors and windows installed.
- Rodent infestation can be a problem. Straw bales may arrive containing grain-eating insects, such as flat-grain beetles, saw-toothed beetles and merchant-grain beetles. Factors that influence the severity of the infestation are the presence of grain dust mixed with the straw, its moisture content, how long the straw was left out before it was baled, and where it came from.
- Due to misconceptions and their unconventionality, load-bearing straw bale homes (not post-and-beam homes that use straw bale insulation) may pose the following difficulties for homeowners:
- The house type is not accounted for in some building codes, which can make it difficult to obtain a building permit.
- There may be a lack of acceptance by the local neighborhood based on the home's comparable aesthetics and value.
- There may be difficulty in obtaining financing by conservative banks and lenders. Prospective straw bale homeowners should be prepared to explain the construction to banks and lenders who are unfamiliar with the approach. Consultants may need to be hired to inspect the building or its plans and vouch for the owner to lending institutions.
- It suffers from a weakened real estate value. Straw bale homes are often regarded as inferior, experimental or risky, and thus are not as readily trusted by prospective buyers.
- There may be difficulty in obtaining homeowners insurance protection. For the same reasons that they’re not easily financed, insurers may be reluctant to insure these buildings.
- They are susceptible to moisture-related problems. Damp straw will decay, weaken as an insulator, and encourage the growth of mold. Moisture levels, which should be at or below 14%, can be tested through the use of a moisture meter. Moisture may originate from the following sources:
- cracks in the plaster. These cracks must be repaired promptly. If moisture enters the bales, it will stay trapped beneath the plaster;
- plumbing pipes routed through the straw. Any leak or condensation from the pipes will bring moisture to the surrounding straw;
- missing or damaged flashing;
- inadequate roof overhangs;
- inadequate clearance from grade to the bottom of the plaster wall at the exterior. Twelve inches of clearance is typically adequate; and
- windowsills and joints not properly sealed. Windowsills should be sloped to drain water away from the window opening.
Straw bales may also become moist before construction is completed, especially if they weren’t stored in a dry, protected area prior to or during construction. It is critical that builders inspect straw for moisture before it is used.
Additional Construction and Inspection Tips
- Straw bales should be anchored to each other so that they don’t topple over during construction. Stakes of wood, bamboo and rebar are commonly used for this purpose.
- Straw bales provide the most structural strength when laid flat, although they may also be laid on edge. However, the service life of the plaster bond for bales laid on edge will be shorter.
- Bales should be strung together firmly and tightly with either twine or baling wire.
- Load-bearing straw bale houses should be allowed to settle for some time before stucco is applied.