[Cob] Moisture, mold, and walls

[Shannon C. Dealy]

On Sun, 25 Jul 2004, GlobalCirclenet wrote:

[snip]
> mold problem it's because the house isn't properly sealed against moisture
> where it should be sealed, and you have to move out. Then it costs a
[snip]

This isn't correct, while a lack of seal against moisture could cause mold
problems, it is in fact buildings that are to tightly sealed which are the
ones making it into the news and are the source of the reports on "sick
building syndrome".  I have looked at the facts surrounding quite a few of
these, and in each case, the wall system sealed the house so tightly that
moisture could not escape, and since living is an inherently moisture
producing activity, moisture levels shot up in variouss places in the
walls and interiors of these buildings, ultimately reaching the point
which would support mold growth.  Cracks in interior plasters on bales
isn't going to create a mold problem in and of itself (though in wet
climates exterior cracks could), it also requires a source of sufficient
moisture concentrating in that area, so cracks in a bale plaster job in a
bathroom is far more of a concern than plaster cracks in a living room.
The problem is not that moisture gets into walls, regardless of what
they are made of (bales, cob, wood frame, concrete, steel, etc.), IT WILL,
the problem is does the wall system allow the moisture to escape from any
and all areas on average as fast as it infiltrates.  It is important to
note here that by escape, I mean pass through the wall, since it must be
able to reach the side of the wall with lower humidity, or it hasn't
escaped.  Cracks in interior plasters on bale walls allow more direct
contact with the interior air of the building, but this is a relatively
minor issue unless that air has extremely high relative humidity (such as
a bathroom or possibly kitchen), in which case, even a straight earthen
plaster over the wall may have issues as well, and may not be sufficient
to protect the bales behind them.  Moisture is going to get into the wall
through even a perfect plaster job, it is even going to condense in the
wall (bales or otherwise), but as long as it is able to migrate out
reasonably quickly by wicking, air migration, or other means, mold is not
going to be a problem.

How moisture gets into and out of walls is one of the most important
lessons of modern building, because a large percentage of building
officials, architects, and building material manufacturers do not
understand this most basic of problems, and in the interest of higher
insulation / energy efficiency, reduced maintenance, etc., make the
absolutely worst possible design choices.  Some of them have learned from
these mistakes (based in the recent increase in articles regarding
moisture and wall breathability), many are still clueless.

[snip]
> will mold faster than wood. Cob or other earthen material will not mold.
[snip]

It might be more correct to say that the interior of cob or earthen
materials will not mold (where the interior of straw bales definitely
will mold), the straw sticking out of cob as well as the organic matter in
the soil the cob was made with will mold on it's surface, even after the
cob has dried if the relative humidity is high enough (and yes, I do know
from personal experience -- I sometimes wonder if I don't experiment far
to much :-)  Mold will actually grow on anything, since even the dust in
the air has sufficient organic matter to support mold, all it has to do is
settle somewhere moist, this is how galvanized steel ducting in houses and
office building is able to support the growth of mold.


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com