Cob: RE: RE: Insulation for cob

[Michael Saunby]

On 08 July 1999 16:04, Kelly, Sean [SMTP:SKelly at PinpointTech.com] wrote:

> post, cob is just clay, sand, straw and water.  The strength comes once
> it is dry.  The clay acts as the mortar or glue, the sand (needs to be
> angular, not rounded beach sand) acts kinda like interlocking bricks,

I know the new cob builders use rather different mixes but traditional cob 
(once it's gone off) is earth, straw, and some moisture.  The earth is of 
course a mixture of stones (about 1 inch or less) down to sand (though not 
much where I am) and clay.  It's also go a lot of fine gaps in it, since if 
you add water the mass increases considerably but not the volume (a simple 
test of such things). The straw acts not just to hold it together but also 
to hold it apart, reducing the external shrinkage.  Adding lime also 
reduces shrinkage and probably has a reasonable history so your wall might 
even last (if that matters).

In the end it's like the stuff about newspapers earlier this week, most 
solids are poor insulators, it is usually the air trapped inside or between 
layers of the material that keeps heat in or out. Rather than adding funny 
stuff all that's really needed if you want good insulation is lots of fine 
bubbles, which would also reduce the volume of materials needed.  How you 
do that I don't know.  Even so, it's always much better to start with basic 
physics, chemistry and engineering than alchemy.  Just chucking something 
in because it has a certain property is a waste of money, you usually find 
that it's the method of use that gives it the property not some magic 
contained in the material, even newspapers or earth :-(

Michael Saunby