R-Value of Straw Bales Lower Than Previously Reported

Much of the published information on the energy performance of straw-bale buildings is based on measurements done in 1993 by Joseph McCabe at the University of Arizona as part of his master’s thesis. McCabe used a “guarded-hot-plate” apparatus (procedure ASTM C-177-85) to measure heat flow through a single bale that was 23” (580 mm) wide by 16

1⁄2” (420 mm) tall. He reported R-values of 48.8 (RSI-8.6) for bale on edge and R-54.8 (RSI-9.7) for the bale laid flat. Thus he concluded that the insulating value is R-2.68 per inch (0.054 W/m°C) when heat flow is perpendicular to the orientation of the straws (bales stacked on edge) and R-2.38 per inch (0.061 W/m°C) when the heat flow is parallel to the straw orientation. These values were reported in