The new test was designed to overcome problems that distorted straw bale's thermal performance in other tests. Staff at Oak Ridge National Laboratory (ORNL) conducted the test in their hot-box test chamber. The wall being tested was built using 19-inch two-string wheat straw bales laid flat and stuccoed on both sides. After a two-month drying-out period, the wall was put in the test chamber. The interior temperature was raised to 70°F, while the exterior temperature remained at 0°F for two weeks, in order to reach steady-state heat flow conditions. After this two-week period, the 19-inch wall had an R-value of 27.5, or 1.45 per inch.

In earlier tests, the R-values of walls constructed with 18-inch straw bales had ranged from 17 to 48 (see Table 1). The higher value resulted from a test of straw bale conductivity using a thermal probe. The lower value was thought to be due to air gaps between the gypsum board used to surface the warm side and the straw bales.

When weighing what R-value to use for modeling the performance of straw bale homes, the California Energy Commission gave the greatest weight to the most recent ORNL test but decided to use a slightly conservative R-value of 1.3 per inch (or R-30) for 23-inch walls. Using this value, the commission calculated total annual heating and cooling energy savings from building with straw bales in five California climate zones. The savings ranged from 12% to 22%, with an average energy savings of 17%.

Home Energy can be reached at: contact@homeenergy.org
Home Energy magazine -- Please read our Copyright Notice