[Cob] strawbale, geodesic domes, and igloos

[Shannon C. Dealy]

On Thu, 22 Jul 2004, GlobalCirclenet wrote:

[snip]
> strawbale with cob. It's clearly about thin coats of plaster, not cob at
> all. It recommends earthen plasters. The difference is significant because
> cob is not the same as earthen or any other kind of plaster. Plaster is
> applied in very thin coats, esp the first coat to "key" into the straw.

That is one recommendation, there are many approaches to plastering bales,
cob works perfectly well, and "earthen plasters" are in fact simply cob or
close enough variations as makes no difference (cob is a particular type
of mixture, not how thick you apply it).  I have personally applied
cob type plasters in both thick (a couple inches) and thin coats to straw
bales, dating back about nine years, and have worked with many people
who have far more experience than I do.  There are no problems with doing
this, though it might be a concern if it were done in cooler weather when
the drying time might be slow enough to cause problems (say mid-October to
mid-April here in the Willamette valley of Oregon).  The primary reason to
use thin coats is that multiple thin coats will finish drying faster than
one thick coat, and it will take less plaster to reach a fine crack free
finish layer.  Of course if you want more thermal mass, are working in
warmer dry weather, and it will dry faster than you will be ready for the
next coat, it takes alot less work to go with the single two inch layer
for a base coat.

[snip]
> afford the contractor liability. Most likely because cob has to be much
> thicker than plaster, and takes longer to dry, and releases too much
> moisture into the straw.
> Plain cob is a safer and much more interesting material.  Sealing straw
> properly against moisture means paying professionals for a lot of labor
> plastering. You simply cannot risk health problems later by experimenting
> or learning on the job. It is not for first time do it yourselfers. Whoever
> said strawbale was fast, easy, or cheap forgot about the horrendous job of
> plastering completely.

Cob is not defined by thickness, and does not have to be "thick", it is
perfectly safe to use on bales and has been widely used for this purpose.
I have made cob plasters using EXACTLY the same mixture that went into cob
walls, the only difference being that the sand and clay were screened to
remove lumps and the straw was shorter (same total volume of straw, just
shorter) though many people use full length straw these days in plasters.
Adding extra water may be desirable in many cases to make the mix more
workable, particularly with longer straw, but this depends on
circumstances and is not always a necessity.

As far as straw bale being:

   fast - it takes roughly the same amount of time as wood frame
          construction to do a comparable structure, which isn't fast but
          is certainly faster than cob.  Much faster bale structures have
          been built (one small one that I took part in was done in a day
          excluding plaster), but these generally are not designed as full
          blown houses.

   easy - it isn't particularly difficult for load bearing walls, for
          bale infill, someone needs more serious carpentry skills.

   cheap - depends on the design, availability of materials, and how much
          labor you do yourself.  Like cob it can range from $0 with lots
          of personal labor, on up to as much as you would like to spend
          and beyond.

It is important to be careful not to read more into what people say on
the internet than is really there, as sometimes the information is not
well worded, or poorly thought out, depending on the circumstances under
which the person wrote it.  I have personally made some glaring errors in
postings over the years, and if someone read just that posting and not the
follow up where the error was corrected, they might come to the wrong
conclusions.  There is also another danger in that some people post
opinions based on other people's opinions, which may in turn be based on
other people's opinions, etc. and there may not be any actual experience
behind the statements.  I cannot say if either of the above apply to the
source for your statements, but there is lots of real world experience out
there that says cob plasters on bales is just fine, and it has been done
for at least 9 years that I am aware of, so problems in the buildings I
worked on should have shown up long ago.


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com