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Re: GSBN:Re Thermal Properties of Straw Bale Walls



Straw-clay seems the best material for this purpose, although too much clay
might increase conductive heat loss in the stuffed area. <

...for several other reasons, too.  When straw is stuffed in joints it's
relatively loose and so provides a ready channel for moisture into the
depths of the wall.  Straw-clay will form a seal against moisture entering
the joints between the bales.  In addition, it stops air infiltration and
can be screeded off to make a flat plain for plaster, thereby reducing the
occurance of cracks that result from abrupt changes in the thickess of the
plaster.  Also, it's fun to get dirty....

"Hurlen" John Swearingen

On 5/19/07, Martin Hammer mfhammer@... wrote:

Nehemiah -

Good explanations.

You talked about the straw-stuffed gaps at the top and sides of a wall as
being places where convective losses could occur.  So I'll add the thermal
importance of stuffing vertical joints/gaps between bales for the same
reason of limiting convective losses.  (This would also be important
between
bales and "posts" that go mostly or all the way through the thickness of
the
wall.  I've seen I-joists or steel trusses used this way.)

Straw-clay seems the best material for this purpose, although too much
clay
might increase conductive heat loss in the stuffed area.  Horizontal
joints
between bales don't seem to be an issue because the weight of the bales
appear to cause the surfaces to lock in well enough to limit air movement
between them (although the French dipped bales might seal that joint even
better, and bales on-edge probably nestle together better than laid-flat).

Then there's always the question of what material is between the bottom of
the plates, and what the insulative qualities of the roof bearing assembly
are.  And then there's the ceiling/roof, and the windows/doors, and the
amount of infiltration throughout, and . . . . .

Martin Hammer


> 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@...







--
John Swearingen
Skillful Means, Inc.
Design and Construction
www.skillful-means.com


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