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Re: GSBN:Re Thermal Properties of Straw Bale Walls



John,

I did not see a response from Andrew.  Was that off list?  I am always
interested in what new or other information people have on the thermal
properties of straw bale construction.  Care to share his input?
Also, your Q about how compaction affects R-value is a potent question.  If
hot box testing wasn't so expensive and time consuming, or if there were
funders lined up to pay for it, I'd already have an answer for you.  There
are a number of confounding factors, so until someone has done the actual
research, we can throw around lots of theories.
For example, it is air that creates the insulation value of almost
everything used for wall insulation.  ,,,not the spun glass, not the solid
portions of the foam, not the cellulose, not the straw.   Therefore, if
bales are compacted too much, one would expect the insulation value to go
down.  But, what is "too much?"  If bales are too loose, then the air can
circulate in the air pockets and research HAS shown that this can lead to
convective currents that lead in turn, to a dramatic drop in R-value.  That
was one of the causes (we think) for the relatively low R-values in the ATI
lab tests in Fresno, CA.  Once we stacked the bales in the hot box wall
opening, and compressed them as they'd be in a building wall, we had a six
inch gap at the top.  We filled it with straw as tightly as we could, but
we are not match for either a baler or truckers' strap tightening levers,
so we KNOW that the top (where the greatest amount of heat exchange would
naturally occur anyway) was much looser than the rest of the wall.  Ditto
the sides, though those gaps were significantly smaller (so perhaps, harder
to compact straw into).
Further, though in theory greater compaction - after the optimal point -
will lead to a decreasing R-value, no tests have yet shown that to be the
case.  Perhaps we just haven't found the optimal compression force yet.
Perhaps the theory is wrong.
Lastly, I would question your assertion that jumbo bales are "naturally
compacted a lot more than the smaller bales."  It is mechanically more
difficult to compact a larger bale to the same density as a smaller one.  I
am not saying that the machinery isn't designed to do so - perhaps it is.
But, from a pure physics point of view, it is not "natural" as you said.
One way to verify whether the compaction is greater or not is to measure
the water content and density.  Rice straw bales in California (the ones we
tested) are typically at least 8 pounds per cubic foot at a moisture
content of about 6%.  Do you have similar data on the Aussie jumbo bales?
If you want to get an accurate reading of the moisture content (more
accurate than a moisture meter stuck a random depth into the bales), let me
know and I will send you (offline) a description of how we did it.  The
density (#/cf or kG/cM) is pretty easy, assuming you can weigh a
representative sample of the bales.
Hope this helps.
Thanks,

Nehemiah Stone
stoneandstraw@...



> [Original Message]
> From: John Glassford jacksflat@...
> To: GSBN GSBN@...
> Date: 5/17/2007 7:18:59 PM
> Subject: Re: GSBN:Re Thermal Properties of Straw Bale Walls
>
> G ' day Nehemiah and Andrew et al
>
> Many thanks for the replies and for your considered input.  It is very
> much appreciated.
>
> I am sure that there is much more work to be done in this area
> especially with jumbo bales as they are naturally compacted a lot more
> than the smaller bales.  Does that provide more or less insulation?
> Many questions ahead of  us.
>
> Kind regards John Glassford.

>