Straw Bale Construction
What It Is
Straw bale construction is a building method that uses bales of straw (commonly wheat, rice, rye and oats straw) as structural elements, building insulation, or both. Building with straw bale offers an array of advantages:
- the renewable nature of straw
- cost efficient
- easily available
- naturally fire-retardant
- high insulation value
- breathability for superior air and moisture control
Overall, a typical straw bale wall is roughly three times as efficient as conventional framing. Over the life of a typical thirty year mortgage, this superior insulation can reduce energy costs by up to 75%, saving money and vital natural resources.
Fire-Retardant Quality
When people ask how straw bales are naturally fire-retardant, the answer can be explained like this: Canadian and U.S. materials laboratories have found that: “The straw bale/mortar structure wall has proven to be exceptionally resistant to fire.” In these tests, the flames took more than two hours to penetrate the plastered bale walls. Conventional framing built to commercial standards took only 30 minutes to one hour to burn. Due to their tight compaction, bales contain very little oxygen and thus resist combustion. It’s like trying to burn a phone book. Loose straw, however, is at risk for fire and should be cleaned from the job site daily. Walls should be plastered as early as possible to increase their fire resistance.
What About Pests?
As for pests, once the walls are properly plastered or sided, there is no way for bugs or rodents to get into the bales. If pests were to get inside, they would find it almost impossible to move in the densely packed bales. Termites and other pests pose more of a problem to conventional construction than they do to straw bale structures.
Moisture Concerns
Moisture concerns can be handled easily with proper design and construction methods. So long as the bales are installed when dry (moisture content of 14% or less) and are properly sealed within the plaster and protected from water infiltration, they will perform well. Even if a building is heavily rained on, it will be fine as long as it is allowed to dry out. Problems arise when high moisture levels (above 20%) are sustained for long periods of time.
Ultra-Durability
Bale homes built in the 1800s still exist in Nebraska and Europe. Straw bale homes have consistently withstood severe weather and wind in Wyoming as well as major earthquakes in California. Many architects and engineers consider straw bales to be the ideal “seismic-resistant” building material. In wind tests, bale structures see no movement in a sustained 75 mph gale and only 1/16 inch movement with 100 mph gusts.
Breathability
Bale walls do not breathe in the sense of allowing air to move through the walls. In fact, it is a good idea to limit air infiltration into the walls as air carries moisture. When we say they breathe, we are referring to the movement of moisture through, and out of, the walls. It is critical that moisture moves out of the walls. Permeable plasters such as Lime or Earth should be used in wet or humid climates and Cement based plasters should only be used in dry climate climates, if at all. This transfer of moisture improves indoor air quality without sacrificing energy efficiency and keeps the bales dry. Straw is a natural material, and as such it does not off gas the way conventional insulations do, adding to the health of the home and its occupants.
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