Subject: [Cob] even the professionals make mistakes

[Barbara Roemer and Glenn Miller]

Copper, maybe it was just a typo but you wrote, "...(of) straw bale
information.  For someone who desperately needs a method of insulating
cob, yes I have built with it, yes I love it, thank
you...but for all practical purposes it needs insulation .."

Since straw is insulation, I'm not sure what you meant.

And Jenn says, 

"... my reading suggests that the mold problems with bales happen when
proper ventilation can not occur and/or excess moisture is being wicked up
through a foundation and if you take precautions with the foundation
and are using a natural plaster some of those risks are minimized as
excess moisture can leave the walls...."

Jenn is right about what she's written.  Bales are really not much different
than conventional walls in this respect.  They need to be protected by a
good "hat and boots," language used by cobbers and balers alike.  What
follows is some points on the foundation and roof, as well as on the
permeability of plasters.  Moisture can wick up concrete some eight stories,
so you must protect conventional stick framing or cob or bales from that
source of moisture.

If a water supply pipe breaks or the washing machine overflows near a bale
wall, you will appreciate the fact that most bale builders, professional and
amateur, build with a "toe-up," which means the bales are a few inches above
the finish floor level and water can reach a drainage plane.  If there is
water vapor being driven into bale walls through the permeable interior
plaster (say from folks, wet winter clothing, showering, cooking, etc.) and
the exterior plaster is less permeable (e.g., cement stucco), the vapor can
condense on the cooler surfaces within the wall.  If the plaster skins are
of the same permeability, it's much less likely to be a problem in moderate
climates.  There is a veteran professional bale builder in Southern CA who
leaves the top of his bales open to the attic space and vents that space
well.  He uses lime/cement exterior stucco and has had fine results with dry
bales.  If both interior and exterior plasters include earth in significant
proportion, moisture in the form of water vapor can move out of the wall.
The equation changes in very humid climates (because the water vapor is
likely to be on the outside of the wall), but as in any humid climate, the
key is getting air flow and moving dampness out of the house, cob, bale,
stick, whatever it is.

What is true of the foundation is also true of the roof: all homes need to
be protected from roof leaks or major damage will result.  Straw bale is no
exception.  Further, it's important to keep the walls from remaining wet, so
appropriate overhangs, wrap around porches in climates with wind-driven
rain, and protection for high gable walls are important considerations.

As far as plaster goes, yes, it's important, particularly for preventing
fire and excluding critters.  But it's not an odious yearly task.  None of
the bale homes I know of are plastered - or even lime-washed - yearly.  A
properly done plaster job (easily done by an amateur with good directions)
should have few small cracks and no large ones, and is good for years.  If
it's earth or earth/lime plaster, it's easy to repair if you do have cracks
or damage.  Of the adobe/cob/bale plaster question, maybe someone else has a
source, but I think I've read that the adobe church at Taos Pueblo is
rendered yearly (that's after the cement stucco used to "restore" it failed
miserably and folks learned that adobe needs a permeable skin).  In
contrast, many bale homes in the area of Obregon in Mexico, are
lime-plastered exteriors and are only redone as needed every few years;
then, a lime wash suffices.  And the calcium chloride is a local waste
product, easily colored brightly for the sizzling color preferred by locals
in that area.

Again, your climate and exposure to weather is very important.  Some
examples of local choices in a building we are putting up some of you might
find interesting.  We have 65 inches of rain in the Sierra Foothills where I
live, but none of it is wind-driven.  We have both a heating and cooling
climate, in about equal measure.  The load bearing bale generator shed we
are about to plaster will be lime/clay on the outside because the overhangs
are minimal, and earthen plaster on the interior.  Warm air from the
generator running in the winter (when we have less available PV power) won't
cause vapor to collect in the bales because it can escape through the
exterior walls.  The plasters will be thick - cob, for all intents - for
sound absorption and for thermal mass.  Temperature is important since the
batteries function best when they are neither too hot nor too cold.  There
is a masonry block toe-up one block high with gravel drainage plan between
the inner and outer sill plates, so the bales are above any water source.
The floor will  be a pounded Japanese style clay with a very small
percentage of lime and of magnesium chloride, and can be hosed out.  It's
over gravel, too.  The building will have a living roof for sound insulation
(that's why the bales, too - since otherwise clay is abundant here and cob
would be a good wall choice), and for water absorption.  The lumber use is
minimal, with site-built trusses and sill plates of small dimension
site-milled lumber.  This is just one building, with choices that are
personal to our site, our resources, the lumber we have available, the
capacity we have currently, the techniques we want to explore, and the time
we have.  The next building will include several different choices, but each
time, we hope the set of choices is appropriate to our place and needs, and
that we learn a lot!

If you are further interested in bales, please contact me off-list.  Seems
as though unless the conversation is about hybrids, and particularly the
intersection of cob and bale, it should be on another list.  Several of you
asked for the Yahoo bale list access.  Go to Yahoo.  Go to groups.  Select
Community and Culture and then search on straw bale.

Barbara