This article was originally published in the March/April 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online March/April 1999

in energy

Refining Straw Bale R-values

Table 1. Straw Bale R-values
  Joe McCabe Sandia Lab ORNL CEC CEC ORNL
Test procedure Hot plate, single bale Thermal probe, single bale Hot box, full wall Approved values Hot box, full wall Hot box, full wall
Test date 1993 1994 Oct. 1996 Dec. 1996 May 1997 Feb. 1998
Type of straw Wheat Not listed Wheat Any Rice Wheat
Type of bale 3-string, 23 in 2-string, 18 in 2-string, 18 in 3-string, 23 in 3-string, 23 in 2-string, 19 in
Moisture content 8.4% Not listed Not listed 20% 11% 13%
Density lb/ft3 8.3 5.2 Not listed 7 6.7 8.0
R-value/ in 2.38 2.67 .94 .56-.91 1.13 1.45
R-value 55 48 17 13-21 26 27.5
Source: Commins and Stone, Tested R-value for Straw Bale Walls and Performance Modeling for Straw Bale Homes, 1998 ACEEE Summer Study on Energy Efficiency in Buildings Proceedings.
A new test of the thermal performance of a wall built with 19-inch straw bales laid flat revealed that the wall had an R-value of 27.5. While this relatively high R-value confirms that straw bale construction can decrease heating and cooling energy usages by up to nearly one-third over conventionally built homes, the new R-value is lower than those that have been previously reported.

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%.

--Mary James



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