[Cob] COB NOT DRYING

[Shannon C. Dealy]

On Wed, 21 Jan 2004, CM Hellwinckel wrote:

> Hello,
>
> I'm building my first cob wall and the first layer on the rock
> foundation is taking over 2 weeks and counting to dry.
> My mix was 60-70% sand and 40-30% clay.and straw.  Its been cold and wet
> as usual in East Tennessee but I have kept the wall covered when
> precipitating.
> Should I just be more patient or is the wetness an indication that my
> mix is wrong?

Drying is going to be slow at this time of year, though you should only
worry about getting it dry enough to take the weight of the next layer.
My experience in Oregon is that a significant increase in drying time
occurs when temperatures start drop into the 50 to 60 degree range and
below.  I have found that the slowest drying layer in the wall is the
first one on the rock base, once the wall gets to about two to three feet
high, drying will go faster.  I speculate that this is because of a
combination of circumstances: proximity to the ground keeps it cooler and
less exposed to wind / airflow and once you have a couple feet of wall up,
some of the moisture from new layers will be wiked into the older / dryer
cob and evaporate off of it's exposed surfaces, giving you more working
surface area.  Having (among other stupid things I have done :-) built
with cob in winter in Western Oregon, here are my recommendations:

    - make sure you cover all surfaces of the cob with finger holes (thumb
      size or use a stick about 1" in diameter), perhaps one to two inches
      deep.  This will give more surface area and promote faster (though
      still very slow drying).

    - If you can't build a roof over the building, rig a cover that is
      suspended well above the cob that you can leave on in rain or sunny
      weather.  You want it suspended above the cob to allow maximum
      airflow over the surface of the cob in all weather.  I found that I
      got the greatest drying on one section of my winter building during
      a heavy rain that was accompanied by high winds, because the cover
      kept the rain off, while allowing the winds to circulate around and
      through the building.  The cover I used was made with string
      reinforced plastic (available at building supply stores like
      home depot) which is very strong, but unlike tarps is clear
      so it allows sunlight in to warm the building below as well as
      giving you light to work by.  If you do rig a cover such as this,
      set it up with a high peak in the middle (like a circus tent)
      otherwise, the condensation you inevitably get under the cover will
      rain on the inside of the building, rather than running down to the
      edges and falling outside.

    - If you are likely to have sub freezing temperatures, you will
      probably want to include sides that you can put on your cover (or
      pull up / lower as needed) in order to trap heat and keep any fresh
      cob on the wall from freezing.

Basically if it's above freezing you want maximum air flow (in hind sight,
if I had power while doing this building I would have brought in some
large fans), if it's going below freezing, try to keep it warm.  I did use
a 45000 BTU tank top propane heater to speed drying in specific areas of
the building (particularly areas that were softened by rain after winds
tore the cover off), but I think over the long haul, a fan running 24/7
bringing in fresh air while temperatures were above freezing would have
been much more effective.

Shannon C. Dealy      |               DeaTech Research Inc.
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