Cob: Re:Cob walls

[SANCO Enterprises <Paul & Mary Salas>]

"Shannon C. Dealy" wrote:

> On Sat, 26 Jun 1999, alexander neumann wrote:
>
> > Hello everyone,
> >
> > Is there anyone who can confirm the prejudice that earthen indoor walls in
> modern airthíght houses make the indoor air too dry for humans and plants?  And
> that the walls get wet over time?
>
> Snipped

> this is silly, the walls start off extremely wet and it can take a year or more
> for a cob structure to dry to a stable moisture level.  The only way for the
> moisture level to start climbing again would be if you have a leak in the roof
> or standing water at the base of the wall.

Shannon, this is partially true.  Our lab test have debunked some of these "lore"
beliefs.  Standing water at the base of a wall will not have a major effect on the
the wall system as a whole.  The capillary absorption of water into the cob or
adobe wall at its base will be dependent on the density of the soil material and
porosity of organic material.  Water tends to wick up the wall only a few inches
above the liquid level to which it is exposed.

Roof leaks primarily erode the surface on which the water is running as opposed to
saturating the wall.  Absorption into the wall in this fashion is very limited
even over prolonged periods. In our area there are numerous untreated earthen
walls that are exposed to the elements all year long and surface weathering is
about 1" per side in 20 years.  We will have lab results on this later this year.

> [SNIP]
> > I can imagine that prejudice coming from old exterior walls  after
> "modernisation".
>
> There have been serious moisture problems inside the walls of old earthen
> buildings that have been treated with modern exterior sealants and plasters.
> These modern materials effectively seal the exterior of the wall, so that
> moisture released into the interior of the building by the inhabitants from
> breathing, cooking, washing, etc., cannot migrate out through the walls, so it
> builds up over time in the wall near the exterior seal, and eventually can reach
> levels which cause damage to the wall.

True, there many recurring failures using modern coating applications, however the
cause in the example is in question. If the above  proposition is true and the
wall had an interior plaster coat, the failure would occur on the interior wall
surface long before the exterior would be affected.  While cement and stucco coats
do fail  the primary cause is a failure in the coating (cracks) as opposed to
vapor/moisture migrating through the wall. The coating failure usually has a lot
of moisture in liquid form trapped between the cob/adobe surface and the exterior
coating (usually cememt/stucco).  Failures caused by moisture created by breathing
and cooking, etc. is nearly impossible because in order to have an affect on the
exterior surface, the entire wall would have to absorb sufficient liquid moisture
to condense on the exterior coating.  Here again if the belief in breathable
interior/exterior coatings hold true, the vapor should migrate right through the
exterior coating without causing damage to the wall or coating. However, if the
conditions in the example were to happened there would probably be a collapse of
the wall.  At about 11% moisture content in a cob/adobe wall,  the material holds
its form and does not exhibit signs of moisture on the surface although there is a
distinct color change and expansion is dependent on the amount of clay content in
the base material. Detrimental effects if any are equal on both wall surfaces. At
about 12.5% to 13.5% the material is nearly saturated, becomes pliable and
moisture is visible.

>From our experiences, once the moisture source is removed, capillary moisture
migration toward the more dense and dry areas slows dramatically and moisture
evaporation reverses and  the more saturated surface dries quickly from the side
that the moisture was introduced.  Continued moisture penetration into the dry
area is minimal. These findings are based on untreated and unplastered or stuccoed
cob/adobe surfaces.

Paul Salas
SANCO Enterprises, LLC
Albuq., NM