Cob: Cob/light clay hybrid

[John Fordice]

Karen & all,
It is important to bear in mind a couple of things when thinking about
cob and insulation:

Cob is essentially a low strength material, and it gets its structural
capacity by being built as THICK and INTEGRAL walls.  The splitting of a
cob wall into two separated or honeycombed vertical wall layers will
compromise the structural capacity unless both layers are of sufficient
thickness.  I surmise that traditional builders didn't make their walls
thick just because they liked to work.  Always remember that traditional
earth wall construction evolved as thick walls for a reason - because
the buildings with thick walls were the successful ones.

Making cob walls the requisite thickness is a bloody LOT OF WORK.  Why
build two thick and thus stable walls of cob when you only need one ?
If you want / need to insulate your walls, do so in a manner that makes
it easy for you ( 1.e. less work ) and that works with the materials you
are using.  Put your cob on the inside where it will do the most good as
a thermal mass to store the precious interior heat you need, and put
your insulation on the outside where it will serve to keep the heat from
being lost by both radiation and convection.

A simple method to do this is make a wide enough foundation to allow you
to wrap the exterior heat loosing walls (East, North & West in the
Northern hemisphere) with stacked up straw bales (relatively inexpensive
& ready / easy to use) and if you want / need to protect the bales,
simply plaster with an inch or so of mud.   Simple, easy to do, easy to
repair, low cost, & structurally sound if correctly done.

Don't make things more complicated than the need to be.

john fordice
maker of cobbers thumbs
TCCP


Roxboro Yurt wrote:

> This isn't a very scientific answer, but I don't think
> this is a very good idea.  Seems like thin hollow
> walls would be  more prone to cracking (and cob is so
> heavy it needs a thick base to hold up the top) and
> any water damage would have much more severe
> consequences (if your thin wall washed away anywhere
> the whole thing might crumble)
>
> The only way I think this might work is if the cob was
> honeycombed so that the two sides were connected in
> many places but the straw or light clay could  fill in
> the holes.  Also - I don't know if you would actually
> want to tamp the straw - air is a good insulator and
> the more air between the straw pieces the better
>
> --- "K. Clouston" <dognyard at worldgate.com> wrote:
> > Has anyone tried building a cob house building
> > essentially hollow walls
> > (with cob inside outside, letting them dry/cure,
> > then tamping the hollow
> > middle with light clay or slip straw? If so, how
> > thick would the cob
> > walls need to be in order to withstand the tamping
> > of the slip straw?
> >
> > I'm wondering if this could, in essence speed up the
> > drying/curing time
> > of the walls. I live in a cold climate with
> > potentially severe winters
> > and would like to find a workable solution to the
> > problems of
> > freezing/thawing of uncured cob.
> >
> > Likewise, has anyone considered building large
> > bricks of slip straw (not
> > as large as straw bales, storing them until cured,
> > then using those as
> > infill within cob walls...perhaps in a honeycomb
> > pattern or implementing
> > a light wood frame construction with slip straw /
> > cob walls?
> >
> > Karen Clouston,
> > Alberta
> >
>
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