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Home Energy Magazine Online March/April 1999
trends
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%.
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