Straw bale house under construction

Straw Houses and Fires

DonanEngineering Fire Investigation

A few months ago while looking through a magazine on green construction, I found an article on houses constructed out of straw

Straw bale constructed house

Figure 1. Straw bale constructed house.

bales. My first reaction was to wonder what happens when there is a fire. However, after completing the article and doing some Internet research, I discovered what would seem impossible: the houses can survive and in some cases can out-perform conventional building materials.

Research into the matter revealed that people have been building houses using straw, grass, or reeds throughout history. In the African prairies, straw houses have been built since the Paleolithic period. In Europe, many houses built with straw and reeds remain standing some 200 years later. Within Germany specifically, some straw houses are over 400 years old. During the late 1800s on the American plains, however, straw bale houses were a matter of necessity, as there was no lumber for construction. Now, with the increased attention to sustainable housing, there has been a renewed interest in straw bale construction.

So, just how are these houses built? Well, straw bale construction typically consists of stacking rows of bales on a raised footing or foundation, with a moisture barrier or capillary break between the bales and their supporting platform. Bale walls can be tied together with pins of bamboo, rebar, wood, or surface wire meshes. The walls are then stuccoed or plastered, either with a

Straw bale house under construction

Figure 3. Straw bale house under construction.

cement-based mix, lime-based formulation, or earth/clay rendering. The bales may actually provide the structural support for the building, as was the case in the original examples from the late 19th century. Alternatively, straw bale buildings can have a structural frame of other materials, such as lumber or timber-frame, with bales simply serving as insulation, which is most often required in northern regions and/or in wet climates. A combination of framing and load-bearing techniques may also be employed, referred to as “hybrid” straw bale construction. Typical bales created on farms with baling machines have been used and can support 600 pounds per linear foot, while higher-density pre-compressed bales or straw blocks support up to 4,000 pounds per linear foot.

All of the different methods have one similarity: the walls are covered with cement, plaster, or clay once the house is constructed. This layer provides a crucial barrier between the straw in the event of a fire. Research conducted by several entities reveals that once flames breach the protective layer over the straw, the fire tends to only smolder. This attribute is related to the density of the straw bales.

Two ASTM E119 small-scale fire tests were conducted in 1993. One of the tests involved an unplastered straw bale wall panel, and the second involved a straw bale wall panel with stucco on the outside face and gypsum plaster on the interior. The results of the testing revealed that the unplastered straw bale wall panel resisted flame penetration for 30 minutes, while the stuccoed straw ball wall panel resisted flame penetration for over two hours. A subsequent full scale ASTM E119 established that plastered straw bale walls amount to a 1-hour fire-resistive construction.

Tested wall assembly with straw bale infill

Figure 4. Tested wall assembly with straw bale infill.

In addition to the above tests, an ASTM E84 flame spread test was conducted in 2000 on unplastered two-string straw bales. The testing showed a flame spread index of 10 and a smoke development index of 350. The 2006 edition of the International Building Code requires a maximum flame spread index of 25 and a smoke development index of 450 for insulation. This means that straw bales easily surpass both requirements and can be used in commercial and residential applications!