[Strawbale] re. straw clay insulation values

Thu May 3 06:52:02 CDT 2007

     The following is from a pdf file sent me by Lou of Design Coalition in 
Madison, WI.    There are two graphs in the report also,showing how 
conductivity varies relative to density.   I visited Lou's clay straw home on a very, 
very hot day last summer, and it was wonderfully comfortable inside.     He has a 
geothermal pump, too.
     Shivani in WI

ENGINEERING REPORT OF LIGHT CLAY SPECIMENS THERMAL CONDUCTIVITIES FOR DESIGN 
COALITIONS STRAW/CLAY FORMULATIONS EXTEND VOLHARD’S K-VALUE VS. DENSITY CURVE 
IN LOW CONDUCTIVITY END FOREST PRODUCTS LAB MADISON, WISCONSIN MARCH 12, 2004

Four formulations of light weight straw loam provided by Design Coalition 
where tested for thermal conductivities with the results as shown in table 1 
below.   The specimens where first removed from their lay up forms and band sawed 
to 4 or 6 inch nominal uniform thickness.   The specimens were then 
equilibrated to 75 deg. F and 50% relative humidity in the room in which they were 
thermally tested in a Lasercomp   Fox 600 series thermal tester.           

Table 1 Specimen   Density Density   Conductivity delta temp. temp_median 
R/inch (#/ ft^3) (kg/m^3)      (W/m*K) (deg. C) (deg. C) (hr*F*ft^2/BTU/inch) Low 
Dens. I 10.2 164 0.08 4.54 23.21 1.80 Low Dens. II 13.0 209 0.09 approx 4.5 
approx 23 1.69 So. Dakota I 15.8 254 0.09 4.51 23.40 1.55 So. Dakota II 13.3 
213 0.09 4.54 23.39 1.67 Reg I 13.3 213 0.08 4.55 21.30 1.72 Reg II 13.7 220 
0.09 4.54 23.59 1.66 NM I 38.1 612 0.13 4.58 23.21 1.11 NM II 43.9 705 0.16 4.19 
23.40 0.90 After thermal testing the specimens were oven dried at 105 deg. 
Celsius and reweighed to estimate their moisture content at the time of testing 
with results shown in table 2 below. 

Table 2 Specimen Test weight Oven dry Moisture content (grams) (grams) (% dry 
wt. bases) Low Dens. I 6878 6426 7.0 Low Dens. II 8652 8121 6.5 So. Dakota I 
10953 10383 5.5 So. Dakota II 9330 8839 5.5 Reg I 8867 8382 5.8 Reg II 9158 
8644 5.9 NM I 17660 17059 3.5 NM II 20379 19774 3.1

The formulation as provided by Design Coalition is as follows:          
STRAW-CLAY INSULATION TEST SAMPLE PREPARATION Three components were mixed to make 
the straw-clay insulation infill for the thermal tests. These were Wisconsin 
wheat straw, brown clay from surface excavation near Madison Wisconsin   (here 
after called Madison clay) and tap water. The straw was in 25 pound bales that 
tested 13% moisture with a range of 11-15%.   Test depths were 4 and 8 inches 
using a wood moisture reading from a Delmhorst BD10 moisture meter. The 
locally mined clay when analyzed was 41% clay, 40.6% silt and 18.3% sand & 0.1% 
gravel.   Straw, clay and water were combined in 1:1:7 weight ratios respectively. 
Clay slip was made first by combining the clay and water. This was done by 
mixing the clay into the water with a Silverson Hydromax TM high shear mixer or 
by soaking the clay overnight in the water then mixing with a paint stirrer on 
a drill. Straw was then put on a plastic tarp and the clay slip used to coat 
the straw by pouring it onto the straw while tossing the straw with forks. 
When homogeneous, the mixture was stuffed in a five sided frame made from 2x6’s 
with OSB sides measuring exactly 2 feet by 2 feet on the interior and the width 
of the 2x6’s. Stuffing was done by lightly stomping on the mixture as it was 
added. When full, the top 2x6 was added and the OSB sides removed after the 
sample had set 2 hours. Narrow wooden diagonal corner pieces were added after 
the OSB was removed to maintain frame geometry. The samples were allowed to dry 
in ambient conditions protected from any direct weather exposure as rain or 
snow. ___________________________________________________________________ 
Conclusions: The light weight specimens provided by Design Coalition appear to 
extend the measured properties of denser formulations of Straw and clay/loam 
materials sited in the building design literature (Minke, 2000: Earth Construction 
Handbook,pp35).   The curve fit to thermal conductivity of the Volhard data 
(Volhard,1983) places materials with densities of 300 kg/m^3 at 0.1 W/mK.     As 
the plot below shows the FPL measurements fall on the trend line of Volhard 
data. Thermal performance at mean temperatures other than the room temperature 
averages chosen for our tests were not measured and may be presumed to change. 
Thermal performance at moisture levels other than those shown in table 2 were 
not measured and may be presumed to change. The weights before and after 
thermal testing were taken with only nominal losses of around 10 grams measured- 
near the accuracy of accounting for straw fiber being chaffed off the specimen.  
 Moisture movement at higher temperature differentials than we used may be 
significant.

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