Strawbale Walls Fare Well in Firewall Tests

The protocol for ASTM E119-05a, Fire Tests of Building Construction and Materials, involves heating one side of an assembly to a defined time-temperature curve reaching approximately 1,850°F (1,010°C) in 120 minutes, then spraying it with a moving stream of water to test the wall’s post-fire coherence and stability.

The first assembly tested—with one inch (25 mm) of cement-based stucco on both sides—passed the two-hour burn test with minor cracking on the heated side. The highest recorded temperature on the unheated face was 166°F (74°C), well below the defined maximum temperature rise failure point. When the assembly was demounted from the furnace chamber (test subjects are built into movable steel frames), the stucco on the burned side—which, during the final hour, had bulged out 10 inches (250 mm), held together by reinforcing mesh—fell off, followed by a self-extinguishing flare-up. The 2 ½–minute hose test (30 psi [200 kPa] at 20 ft [6 m]) was consequently performed on burnt, bare bales, without incident. Subsequent examination revealed the depth of charring into the bales ranged from about two inches (50 mm) to nearly 12 inches (300 mm) at loosely “chinked” bale interstices, and at the corners and along the top of the assembly, with an overall average charred depth of 3–4 inches (80–100 mm).