Cob: cob vs strawbale

[Shannon C. Dealy]

On Tue, 5 Sep 2000,             wrote:

[snip]
> arguments to support their contention.) The cob wall, which can continue
> "curing" for a year and more, could potentially contribute enough
> moisture to the bales to begin rotting them if the entire assembly -
> that is, cob/bales/plaster - is made all at once (assuming that the cob
> is of typical cob proportions and methods, rather than being an
> extra-thick earthen plaster).

I haven't seen any evidence one way or the other on strawbale sandwiched
by cob rotting, but it is important to point out that while it may take a
year for the moisture levels in a cob wall to stabilize, the drop in
moisture is not linear.  Depending on the amount of water in your mix,
humidity, temperature, sun exposure, and how fast you build up the
layers, better than 90% of the water can evaporate in just a few days.
As the water levels in the cob drop, the rate at which it evaporates
slows, so while it may take a year to fully cure, for the overwhelming
majority of that year, the moisture levels should be fairly low.  Of
course when and how you build can greatly affect this, if you build in
cold/humid weather and layer it up quickly, you could experience little or
no drying for many months (until the weather warms and the humidity
drops), so for greatest success with cob/strawbale sandwiches it might be
a good idea to schedule your building for the best drying conditions.

> Straw-clay (a.k.a. light clay, a traditional German timber-frame infill
> material called Leichtlehm, with centuries-old examples
> extant) practitioners typically limit the thickness of their walls to
> not more than 12" because the straw in the center will rot before the
> wall can dry^E and straw-clay presumably dries faster and has less
> initial moisture than cob.
[snip]

While I would expect straw-clay to reach full cure faster, it MIGHT not
follow the same kind of drying curve that cob does (different materials,
different behavior, but I don't know for this case).  In other words,
while it might be completely dry sooner, it might maintain higher internal
moisture levels for a longer period of time than cob.  Regardless of
straw-clay's drying behavior, there is an important difference here,
straw-clay starts out wet all the way through, where a cob/strawbale
sandwich would start only wetting the outer edges of the bales (where it
is joined to the cob).  This has a couple of implications:

   1 - It will take some time for the bale to wick enough moisture
       out of the cob to cause moisture levels throughout the straw
       bales to reach a level sufficient to cause rot (though some
       localized areas could have problems sooner).

   2 - While the straw bales are absorbing moisture, they are also
       drying the cob, so if there isn't enough excess moisture
       in the cob to raise the water levels of all the bales up to 
       a level sufficient to cause rot you may not have any problems.

Of course there is no substitute for trying it, monitoring the humidity
in the bales, and checking for rot afterwards.  Any one have the time and
energy to try it :-)
       
Shannon C. Dealy      |               DeaTech Research Inc.
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