exterior wall protection

[Shannon C. Dealy]

On Thu, 17 Oct 1996, christopher j. meyer wrote:

> 
[SNIP]
> some form of fiber should be tried.  I may be missing the point of the
> discussion in which I noticed some concern for wall breathability.  If a
> wall is dry, what harm is there in sealing it?  If freezing is a concern
> there are certain ways of introducing entrained air to the plaster mix.
> Also, coloring can be added to the p.c. mortar to help with aesthetics.  I
> don't have any hands-on  experience so my approach from a purely academic
> background may expose my ignorance.  Adios, Chris.
> 
The problem with sealing ANY wall is that the seal is never perfect.  Over
time moisture will get into the wall (even a solid cob wall).  Since the
moisture often enters as a vapor rather than a liquid, it can migrate into
the wall fairly easily until it reaches the location in the wall where the
dew point has been reached (assuming winter time here), then the moisture
returns to the liquid state.  Over time, a fair bit of moisture can
accumulate in one location through this mechanism.  Now, the problem: when
the weather changes, the moisture can evaporate but since the wall has
been sealed it can only escape through the few places it entered, and
since moisture is stupid it simply spreads throughout the wall while some
of it slowly escapes through the few spots that it entered.  Since most of
the moisture remains in the wall for quite some time, all sorts of nasty
things can happen depending on what your wall is made of, strawbale is
noted for its FGS (Fetid Goo Syndrome).  Cob is relatively immune to these
problems, but sufficient moisture will weaken a wall.  There are however
other reasons not to seal a cob wall, for maximum strength you want the
cob to dry completely (which can take 1 - 2 years), in the mean time there
is no reason that you can't plaster the wall as long as the plaster
breathes and allows the cob to continue drying.

All of the above, assumes the best, no problems with your building, but
what happens if your roof or window sill or water pipe leaks into the
wall, since your wall is sealed, there is no easy way to dry it out.

One final reason: Most methods of sealing a wall have some rather nasty
environmental side effects, or at the very least use a lot of energy.


Shannon Dealy
dealy at deatech.com