Cob: Re:Cob walls

[Shannon C. Dealy]

On Wed, 7 Jul 1999, SANCO Enterprises <Paul & Mary Salas> wrote:

[snip]
> 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.

I have a real problem with this statement.  When you have cob standing in
water, it will at some point begin to weaken, and at that point it will
begin to act like pudding, all the weight of the wall and roof that are
being supported by that bottom few inches will cause it to be squeezed out
at the bottom, resulting in more of the wall coming into contact with the
standing water and continuing the cycle (the incredible shrinking house
:-)  This may not be much of a problem if just a small section of the
base of the wall is in standing water, since the remainder of the wall
will continue to hold things together and provide the necessary support.
How much water it takes to achieve this effect and how long contact is
required will vary tremendously depending on the mix being used.  The
"ideal" cob mix has a very high sand content which makes it very resistant
to this problem because the sand doesn't absorb the moisture, only the
clay and straw do, so it may take a long time to achieve moisture levels
sufficient to cause failure.  Anyone who has tried to add cob to a dry
wall built with a high sand mix knows that it is virtually impossible to
get the top of the wall to soften by continually soaking it with water,
but if the wall has a high clay and fiber content instead, a few hours of
soaking may suffice to make the top of the wall almost as soft as when the
cob was first applied (this is in fact the case with a demonstration
building I am working on right now).

> 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.

Since everyone's cob mix is different, the effects of capillary action
could vary widely, resulting in water wicking much higher than just a few
inches in some walls.

> 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.

True enough, though my response was only intended to indicate possible
ways in which the moisture level in the interior of a wall could begin
to increase which this would do, though for a large scale leak, I  
certainly agree that the majority of the damage to the wall will be due to
errosion.  The history of cob in the U.K. is consistent with your 
findings, since there are a number of examples unprotected cob walls which
have withstood many decades of exposure to weather without failure (though
of course with significant errosion damage).

> > [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

Actually this would not be the case, since if the sealant coating/plaster 
were placed on the interior of the wall, then the moisture is trapped in
the room, not in the wall.  The failures of this sort that I am familiar
with were due to moisture build-up in the wall at the point where the
sealant is attached to the wall, weakening the coating's bond.

[snip]
> 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.

Not quite, you are not considering temperature swing.  Water VAPOR can
migrate readily through the wall to the point of the seal, and should be
relatively uniform throughout the wall, but at night when the temperature
drops, the exterior sealant/coating will become much cooler than the rest
of the wall (since it is in direct contact with the cool night air),
resulting in the water vapor condensing in the wall, almost exclusively
right at the bond between the external seal and the wall (since only the
outermost portion of the wall will go through significant temperature
change overnight).  This will result in moisture build up at the junction
in much the same way a glass of ice water builds up a layer of moisture on
it's exterior.  While the wall will tend to wick the moisture away, a
daily repeating cycle can cause a build up of water, and it is not
necessary to reach a level which would cause failure of the wall, only a
level sufficient to cause a failure of the bond between the sealant and
the wall, which may be due to disolving of one of the materials involved
in the bond at the point of contact, moisture expansion, or some other
means.

> 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
[snip]
This is true, however the issue I raised was moisture damage due to the
use of non-breathable exterior coatings.


Shannon C. Dealy      |                    DeaTech Research Inc.
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