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 EBN’s feature article on straw as a building material ( EBN Vol. 4, No. 3) and in many other publications.

Follow-up studies conducted since 1993 have given widely differing results. In 1994 a thermal probe was used by R. U. Acton at Sandia National Laboratory to deduce the R-value of a 16 ¹⁄2”-wide (420 mm) bale as R-44 (RSI-7.7), which seemed to support McCabe’s findings, but this is considered a fairly primitive testing procedure. In 1996, Oak Ridge National Laboratory (ORNL) constructed a bale wall that was stuccoed on the cold side and covered with gypsum drywall on the warm side. This test found the R-value to be only R-17 (RSI-3.0). On a per-thickness basis, this is just R-0.94 per inch (0.15 W/m°C). The explanation for this very low R-value, suggested researchers, was that an air gap resulted from the way the drywall was attached to the bale wall; this could have created convection currents in the wall, depressing the R-value.

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