[Cob] FW: Cob Code Research Project, Plus ??'s

[Shannon C. Dealy]

On Wed, 9 Feb 2005, Lance Collins wrote:

> Hi Shannon,
>
> You said inter alia,
>
> >two that were built as post and beam with cob "infill" (which is typically
> >how strawbale is done, and in my opinion a really bad idea for cob both
> >from a structural and environmental perspective)
>
> I started out thinking of a load bearing structure but now think that the
> benefits of 'post and beam' are significant.  i.e. you have a roof to work
> under early in the project.  I'm building in a relatively wet climate.

I also live in a wet climate and crazoid person that I am, build year
round.  Having a roof to work under is a definite plus, and though I have
managed to scrape by with tarps, and large sheets of string reinforced
plastic, it definitely would have been nicer/easier at times if there was
a roof independent of the structure.  One approach that some people have
taken is a free standing post and beam roof structure under which they
build, then they remove the posts (and any other excess materials), to
leave it fully supported by the cob walls once they are done.

> I am planning a steel frame, colorbond roof and earth infill walls for

I have some concerns about the use of steel in this manner with cob
because of the significant difference in coefficient of thermal expansion
between the two materials, depending on how attachment is done, it could
result in the steel cracking the cob/earth infill as a result of
temperature cycling (daily, weekly, or annual).

My primary objections to the use of cob as infill on post and beam are as
follows:

  - Post and beam materials are inherently "linear", which means your cob
    infill is going to be pretty much linear as well.  The result is lower
    inherent stability in the cob and instead relying on the structure to
    stabilize it during an earthquake.  This also means that the post
    and beam structure must be built MUCH stronger (using even more
    materials) since it needs to be able to handle the load of all the cob
    being set in motion by an earthquake.  To a great extent you can
    offset these problems by using alot of cob going around posts to tie
    the wall sections together, making your cob "infill" have curves
    while still following a straight line, and similar tricks (in essence
    making the posts irrelevant), but all it is really doing is creating
    more work, increasing cost, and still leaves you with what I view as
    a built-in natural break in the cob around each of the supporting
    posts.

  - Cob doesn't need it, and being among the lowest environmental impact
    materials out there, going post and beam forces the environmental
    impact of the structure much higher than it needs to be.

Personally if I were going post and beam I would build with bales, it's
better suited to the approach, and I could always use a really thick cob
plaster on the interior for thermal mass.  Anyone ever do an 18 inch
thick plaster job?  :-)

> thermal mass insulated on the outside (a code requirement here).

I am a big fan of external insulation around thermal mass (wish I had it
in the building I'm sitting in), just make sure it is breathable (like
straw bales)

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