Cob RE: cold frame with cob walls.

[Shannon C. Dealy]

On Thu, 19 Feb 1998, Jackie Range wrote:

[snip]
> Last weekend we built a cold frame for our vege seedlings.  The temp was still
> too cold inside so we mixed soil and a couple of handfuls of straw together (wet)
> and made a wall around the cold frame.  It has been raining constantly so the mud
> cannot dry so we covered the walls with plastic so that the mud would not wash
> away.  How many days of sunlight are required to dry the wall?

Sunlight while it can greatly speed the drying process, is not really a
requirement.  Air circulation, humidity, temperature, amount of water
used in the cob mix, and exposure to additional moisture (rain fall or
standing water) will all affect the drying time.  Direct exposure to the
sun can heat the cob and reduce drying time, but quick drying is not
necessarily a good thing.  If the cob dries to fast, there can be problems
with cracking.  When it is hot and dry with a low humidity, the cob can be
hard set in a matter of hours which is to fast since it will crack, and it
will be difficult to add to if you aren't finished.  In this case, it is
best to cover the cob with a tarp to trap the moisture and slow the
drying.  When it is cool, wet, and humid, it may take a week or more for
the cob to really set, and it may be necessary to hang a tarp above the
cob in order to keep the rain off, but still allow air to circulate so it 
will continue to dry.  It is always best to build any cob structure so
that the cob is well above the ground and any standing water (a stacked
rock foundation works well).  If the cob is built in direct contact with
the ground, then the moisture in the ground will be absorbed by the cob
preventing it from drying, or even causing it to deteriorate.

Complete drying of the cob walls in a house can take a year, but it
will have most of it's strength once it has set.

> Did we do the right thing?

That depends on how long you want it to last, whether or not you used a
foundation, how much straw and water you used, and what the composition of
your soil is.

> Our next timid experiment will be to build a very large dog house out of cob.
> Has anyone ever done this?

I keep meaning to build one for our dogs, but I never get around to it
since there isn't much incentive (their current house is still holding
together).

> How does one protect the walls once dry?

To protect the walls, all you need is a decent roof on top with a
reasonable overhang.  It doesn't hurt the walls much once they are set for
some rain to fall on them, most of it will run down the outside, and what
little is absorbed will get distributed throughout the wall, so it will
generally require quite a bit of water to weaken the wall.  The real
danger to cob is standing water at the base or water trapped at the top,
not rainfall.  This is not to say that rain on the outside of the wall 
will not cause deterioration, but it will take a long time (decades)
for it to cause significant structural damage, and this can be easily
prevented through the use of exterior plasters and/or lime wash, and
proper maintenance.

Shannon Dealy
dealy at deatech.com