[Strawbale] R value of light straw clay

[Rob Tom]

On Sun, 12 Nov 2006 09:56:46 -0500, kim thompson  
<shipharbour at ns.sympatico.ca> wrote:

> light straw clay

> Can you tell us where those stats and the later R2-3 that Andrew
> mentions come from.

Kim;

The numbers that I was using to estimate the R-values of Andrew's gable  
ends were extrapolated from the chart in Gernot Minke's "Lehmbau-Handbuch"  
which is available in an English translation as well.

Minke's book lists straw/clay mixes of densities in the range 300 to 2100  
kilograms per cubic metre (18.73 to 131 pounds per cubic foot in Imperial  
units) and the corresponding thermal conductivity values of 0.1 to 1.4  
watts/metre*degree Kelvin respectively (or 0.60 to 9.71 Btu*in/hr-ft^2 in  
Imperial units).  The values for thermal resistivity are of course, the  
inverse of the values provided for thermal conductivity.

I've provided a chart with conversions of the SIU values to Imperial units  
and conversion of the thermal conductivity values to R-values per inch  
(way below).

In July 2005, CMHC released a "Research Highlight" entitled:

     "Initial Material Characterization of Straw Light Clay"  (Technical  
Series 05-109)

on a project done by Joshua Thornton. The project officer was of course,  
Don Fugler (who else ?).

Thornton reported values of:

	Thermal Resistance (per inch):  RSI = 0.28  R = 1.6
	for a mix of density: 647  kg/m^3   40 pcf

	Thermal Conductivity:  (for a mix density of) 642 kg/m^3    0.090 W/mK

[Compare Thornton's value for thermal conductivity with Minke's value of  
0.17 for a mix of similar density (600 kg/m^3) and whose corresponding  
R-value would be R-0.85 per inch, almost half of Thorton's R-value for the  
same (almost) density mix.]

Given that the California Energy Commission only allows something like  
R-30 for a 3-string bale wall (ie only about R-1.3 per inch, not that I'm  
saying the CEC is even remotely accurate)) Thorton's value of R-1.6 for a  
straw/clay mix of 40 pcf density seems more than a little optimistic if  
not unrealistic.

In any case, I think that the values that Andrew cites, (R-2 to R-3 per  
inch for cob are way off and by one decimal place at the very least.

===========================================================
Minke's values converted to Imperial units

Density			Conductivity		Resistivity
SIU		Imperial	SIU	Imperial		Imperial
kg/m^3	lbs/ft^3	W/mC	Btu*in/hr-ft^2	R-val per inch
300		18.73		0.1	0.69			1.44
400		25		0.12	0.83			1.20
500		31		0.14	0.97			1.03
600		37		0.17	1.18			0.85
700		44		0.21	1.46			0.69
800		50		0.25	1.73			0.58
900		56		0.3	2.08			0.48
1000		62		0.35	2.43			0.41
1100		69		0.41	2.84			0.35
1200		75		0.47	3.26			0.31
1300		81		0.53	3.68			0.27
1400		87		0.59	4.09			0.24
1500		94		0.65	4.51			0.22
1600		100		0.73	5.06			0.20
1700		106		0.81	5.62			0.18
1800		112		0.91	6.31			0.16
1900		119		1.01	7.00			0.14
2000		125		1.25	8.67			0.12
2100		131		1.4	9.71			0.10
=====================================================

If the above chart gets screwed up by mail programs etc.
there are 5 columns with 19 rows of values
with the values in the first row being
300 (kg/m^3), 18.73 (lbs/ft^3),  0.1 (W/mC),  0.69 (Btu*in/hr-ft^2) and  
1.44 (R-value per inch).

That should be enough for you to reconstruct the table if necessary.

Years ago on the CREST SB List, John Cruikshank (aka Sunny John) provided  
measured densities for various straw/clay mixes and I quoted and requoted  
that post a number of times whenever the topic of R-values of straw/clay  
mixes came up. Unfortunately, I can't find Sunny John's post or my  
re-quotes at the moment. Perhaps someone else has it on file ?



===* ===
Rob Tom
Kanata, Ontario, Canada
<archilogic at chaffyahoo dot ca>
winnow the chaff from my edress in your reply