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GSBN:Straw Bale Demolition and Analysis



At 07:36 PM 6/25/2006, Juliet Cuming wrote:
Thanks to all of you who helped us this Sunday demolish our straw
bale mudroom.  I think we're all more convinced than ever that Straw
( along with lime and mud and a few other ingredients) can be an
incredible and forgiving building material which survives really
well even in our harsh climate.  We're so happy with what we saw
while demolishing that we'll be using all the same techniques we
used 10 years ago to build our new mudroom and also to build our new
cottage next year.  Oh, and it's fun that all our building debris
can be thrown in our compost pile or directly on the garden!  Thanks
again- Juliet and David

There are a small few before, during, and after photos at
<a  target="_blank" href="http://www.flickr.com/photos/duckchow/";>http://www.flickr.com/photos/duckchow/</a>

We'll put together something more comprehensive about it all later,
but in short: I concur with Julia's synopsis - it was pretty impressive.

This was in many respects a "worst-case" situation. The
weather-receiving, windward north wall of the unheated mudroom (added
to the design in afterthought) had almost no roof overhang; received
splashback accumulated from the gutterless main roof and porch roof
(average annual rainfall, 41"); was exposed to significant snow
drifting every winter; and perhaps most debilitating, had dense
foliage directly against the wall. The mudroom was also an
unconditioned space, so any drying value that might be given by
winter heating and pressure differences was largely absent.

This had been going on for ten years. This year has been wetter than normal.

The mudroom was built on a deck on a pier foundation. Using a forage
probe, we recorded moisture-content levels at various depths in the
lower three feet of the wall on an approximately-six-inch grid of
holes punched through the lime plaster with a piece of rebar and a
three-pound sledge. The general pattern was no surprise, mirroring
previous findings by Kim Thompson, Shawna Henderson, Clark Sanders,
Bob Platts, Rob Jolly, and others (much of that data resulting from
studies undertaken for CMHC, something like the Canadian version of
HUD in the US). Moisture accumulations were highest at the outside
face of the straw at the bottom of the wall, diminishing as height
and depth were gained.

Moisture content at the face of the bales in most of the probe-holes
of the lower 18" of the wall was high, averaging above 30% (ranging
from 20.2 to >40), with diminishing moisture content in the middle of
the wall (averaging about 24%, ranging from 15.7 to 37.6). Farther up
the wall, moisture content at the straw face averaged just below 20%,
with a range from 14.3 to 24.2. The mudroom sidewalls, well-protected
from the weather by roofing, yielded excellent readings averaging about 10%.

We punched probe-holes through the lime plaster at some other
potential trouble spots on other parts of the north side of the
house, and the readings were all less than 15% at the outside of the
straw. These areas have bigger overhangs (though the overhangs are
two stories up, rather than one), no foliage to impede drying
regimes, less splashback due to having less roof area above them, and
(I think) more height above grade than the mudroom had.

When we took off the plaster, there was no evidence of mold - though
the straw at the intersection of plaster-to-bale in the wet areas of
the lower wall was degraded and friable after a decade of these
adverse conditions. The degradation diminished rapidly toward the
interior of the bale, and the straw at an inch or two in was bright,
sound, and strong. The wall system wasn't in any kind of immanent
danger of failure.

A larger overhang, less foliage, and gutters would probably have been
enough to keep moisture content below 20%. A rainscreen system
(basically, siding with an air gap over the plastered bales) would
have prevented the moisture loading of this wall, but wouldn't have
fit the character of the rest of the house.

The new mudroom will have two-foot overhangs and foliage issues will
be avoided. I feel confident that the rest of the house is in fine
shape. Improving the situation, the current building project will add
a covered porch around much of the house, including the entire north side.

On a separate note, I've never been impressed with bale pinning using
rebar or small-diameter saplings/bamboo. (And I'm not alone; exterior
pinning has gained significant favor in the last few years, and
eliminating pinning altogether is increasingly common.) However,
after banging apart the walls David and Juliet put together, I'm
impressed with the method they devised, which involved square oak
pins cinched together with poly twine.


Mark Piepkorn
www.potkettleblack.com
------------------------------------
Be careful of reading health books, you might die of
a misprint.
  - Mark Twain