Cob: new to list, and cob

[Shannon C. Dealy]

On Sat, 1 Jan 2000, Ted Schluenderfritz wrote:

> Thanks Shannon for responding.
> 
> I've read most of the archives on insulating cob and no one seems to have
> actaully tired anything, it's pretty much all conjecture. Have you heard of
> anyone trying to insulate cob? If so did it work?

I believe there are some people who have tried it, but I haven't heard any
specific results.

> Did you ever build the insulated dog house?

No, it wouldn't normally take that much work, but where I am living right
now, the ground is packed with rock which makes it very difficult to use.
The place where I am doing my building work is 30 miles away, and has lots
of nice clay soil - I should probably just bring a truck load home from
the building site to use for the dog house, but rarely drive the truck.

> Do you happen to know anything about the r value of clay/straw infill or have
> any links for more info on this technique? 

I don't know of any links on using clay/straw as an infill, but see my
response to the next question for for a related technique.

> Is there any building technique similar to cob in that it dosn't take special
> tools or a lot of special knowlege to build? I'm deparate to break the wage
> slave 30 year mortgage--highly toxic options.

There is always strawbale, which you can plaster using cob plasters, and
you can use cob for much of the interior work (walls, floors,
benches, etc.).  There is also what is known as "straw-clay", "light-clay",
"leichtlehm", or "leichtlehmbau" (here we go again, I can never remember
how to spell the last two :-) this is a technique where straw is mixed
with a thin clay slip, then allowed to dry until tacky, and then rammed
into a form to create a straw-clay panel.  If this is what you are looking
for, there is some discussion of it in the coblist archives:

   http://www.deatech.com/natural/coblist/

I believe there is also some information in the strawbale discussion group
archives (I don't know where they are on this site):

   http://www.crest.org/

One approach that has been tried in colder climates is to use strawbale
for the North wall (assuming you are in the Northern hemisphere) and
possibly for some or all of the East and West walls as well, then use cob
for the South wall (and possibly part of the East/West walls) which
receives the most sun in winter. This gives you heavy insulation on the
walls which would provide little or no solar gain, and provides you
with thermal mass on the wall(s) which will provide you with solar gain.

If you really want to break out of the rat race, I personally would
probably build small, build round, and use cob.  Start off with a
small studio apartment type design, and design it to allow expansion
if you decide you need to.  Even though cob is lousy insulation, the
amount of energy it takes to heat any structure is proportional to the
exterior surface area of the building (as well as R-value, outside
temperature, etc.), so a well insulated 2000 square foot building will
generally require more energy to heat than a 300 square foot poorly
insulated building (depending on how well/poorly each building is
insulated).  Since cob can be built easily in any shape, going with a 
round building has some major pluses:

   1 - Round buildings are structurally stronger (corner joints act as a
       focus point for stresses during high winds & earth quakes)
   2 - Round buildings can enclose the same amount of space as a
       square, with about 11% less wall (this only affects the amount of
       wall required, it has no effect on roof size).  This means less cob
       and therefore less work.  Rectangular buildings are even less
       wall efficient than square buildings.
   3 - And most importantly to our discussion, since round buildings
       require less wall, they have less exposed surface area, and
       therefore use require less energy to heat.

Quick & dirty examples, assuming all sorts of things including flat roofs 
and 10 foot high walls (to allow for 8' interior ceilings):

  500 square foot building:
    Exterior surface area:
      Roof:              500
      Floor:             500
      Square bldg Walls: 894
      Round bldg Walls:  792 

    Totals:
      500 sq ft exposed to ground (either round or square)
      Square bldg Total: 1394 sq ft exposed to weather
      Round bldg Total:  1292 sq ft exposed to weather

  2000 square foot building
    Exterior surface area:
      Roof:             2000
      Floor:            2000
      Square bldg Walls:1789
      Round bldg Walls: 1584

    Totals:
      2000 sq ft exposed to ground (either round or square)
      Square bldg Total:3789 sq ft exposed to weather
      Round bldg Total: 3584 sq ft exposed to weather

I broke out the floors separately, because during cold weather extremes
(which was the original premise of this discussion) on a uniformly
insulated house (all exposed surfaces having the same R-value), heat
transfer through the floor will be almost meaningless compared to the
other exterior surfaces.  In more moderate weather however, the
floor could become a significant factor.  Hopefully I didn't make any
mistakes here.  If you feel the need to do large buildings, you could
reduce your exterior surface area considerably by going to a multi-story
cubic or cylindrical building.


Shannon C. Dealy      |               DeaTech Research Inc.
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