Cob Plaster on Straw Bale walls

[Bob Bolles]

Hi Tom, and all
Sorry for the delay in responding - the project is taking a lot of my time,
and with no e-mail down there, I can only respond when I'm back in San
Diego.

> Could you give a bit more detail on how earthen plasters are used 
> successfully in areas subject to heavy moisture, i.e., showers.
> 
> Are the shower area earthen walls somehow protected from direct
> contact with the shower water?  If not shielded are the earthen
> walls somehow "fortified" with some form of water resistant
> substance?

Since this is such a dry climate (normally) we don't have much of an
opportunity to see how are buildings will perform under wet conditions.  
Our first showers were simply straw bale enclosures without any plaster of
any kind. After a year of use, we were very encouraged by the fact that
even with repeated exposure to moisture, the bales dried out quickly, and
there was no sign of any mold or fungus.
We then felt that we needed to take the next step and see what happened
with a clay/straw plaster.  The new shower(s) are plastered in the normal
way, and we are experimenting with asphalt emulsion in the finish plaster
to test how well it will shed water.  I will be able to give you results in
a few weeks.
As a side note, the walls on this new shower facility were built in my
absence, and unfortunately, no bottom plate was put under the bales - they
sit right on the concrete pad.  We  are installing a PT 2x4 sill along the
face of the bales sealed with asphalt emulsion in an attempt to keep the
water away from the bottom of the bales - this is the most likely the part
that will fail first.

>From Bill Steen:
Is this a test unit or the real thing?  How much risk are you willing to
take and do you have a 25 year mortgage?  I would truly have to admit that
you are venturing into territory I probably wouldn't consider.  Athena and
I debated several months before deciding on a water-protective coating for
a cob shower that we had built.  Strawbales?  Whose to say.  Lets put it
this way, anything could be possible, but you might want to try a test unit
on the side before going for the real thing in your house.  Probably the
best I could offer is to say that we protect our earthen plastered exterior
straw bale walls with good overhangs, porches, etc. here in Arizona.

Bill, Most of the facilities that we are building now are "temporary"
facilities - how long temporary is, I don't know.  This month we will begin
building more permanent structures, using the information that we have
gained from our experimentation.  If the El Nino condition that is
developing now brings the rainy conditions to the West Coast, as is being
predicted, then we will have a lot more information to draw on as to what
works and what doesn't.  I'm not ready to build a SB shower with clay/straw
plaster in a permanent structure (mortgage or not) until I know a lot more.
 This is my way of learning about wet conditions in a dry climate.

Bob