[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: GSBN:Re Thermal Properties of Straw Bale Walls

An other very effective way to create a snug fit arround the posts was
shown at the last ISBBC: Put a piece of lumber (that has the dimension
of your post) on the ground. 'Drop' your bale onto it .Cut of the excess
straw that sticks out on the side oposite of the piece of lumber. "Et
This trick only works for bales layed flat and with posts that are
moderate to small in size. I forgot who showed us this 'gem' during the

Marty suggested that the 'French dipped' bales might give a good tight
fit between the bales. My experience is the opposite. If the bales are
dried after being dipped the diped sides get rock hard and do not
compress. If the bales are placed before they are dried there is more
chance for settling but this is a messier way of going about.

As for the test Nehemiah mentioned where the top was stuffed. A
suggestions (if such a test is ever to be repeated) is to compress and
stuff the wall from below with car jacks between the 'foundation' and
botom plate'.


André "drop that bale" de Bouter

John Swearingen a ?crit :
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
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
occurance of cracks that result from abrupt changes in the thickess of
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
importance of stuffing vertical joints/gaps between bales for the same
reason of limiting convective losses.  (This would also be important
bales and "posts" that go mostly or all the way through the thickness of
wall.  I've seen I-joists or steel trusses used this way.)

Straw-clay seems the best material for this purpose, although too much
might increase conductive heat loss in the stuffed area.  Horizontal
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
between them (although the French dipped bales might seal that joint
better, and bales on-edge probably nestle together better than

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
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
> 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
> 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
> portions of the foam, not the cellulose, not the straw.
Therefore, if
> bales are compacted too much, one would expect the insulation value to
> down.  But, what is "too much?"  If bales are too loose, then the air
> circulate in the air pockets and research HAS shown that this can lead
> 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
> lab tests in Fresno, CA.  Once we stacked the bales in the hot box
> opening, and compressed them as they'd be in a building wall, we had a
> inch gap at the top.  We filled it with straw as tightly as we could,
> we are not match for either a baler or truckers' strap tightening
> so we KNOW that the top (where the greatest amount of heat exchange
> naturally occur anyway) was much looser than the rest of the
wall.  Ditto
> the sides, though those gaps were significantly smaller (so perhaps,
> 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
> case.  Perhaps we just haven't found the optimal compression force
> Perhaps the theory is wrong.
> Lastly, I would question your assertion that jumbo bales are
> 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
> But, from a pure physics point of view, it is not "natural" as you
> One way to verify whether the compaction is greater or not is to
> the water content and density.  Rice straw bales in California (the
> 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
> 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
> know and I will send you (offline) a description of how we did it.
> 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@...

For instructions on joining, leaving, or otherwise using the GSBN list,
send email to GSBN@...HELP in the SUBJECT line.

John Swearingen
Skillful Means, Inc.
Design and Construction

--- StripMime Report -- processed MIME parts ---
 text/plain (text body -- kept)
For instructions on joining, leaving, or otherwise using the GSBN
list, send email to GSBN@...HELP in the
SUBJECT line.  ----