[Cob] The straw in cob does not insulate!

[Shannon Dealy]

On Wed, 21 Jul 2010, Ocean Liff-Anderson wrote:

[snip]
> R-Value aka "Insulative value" is a measure of the ability to trap heat 
> within the building.  R-value is achieved ONLY through trapped air - in 
> something such as fiberglass, wool, strawbales, foam panels, etc.  The 
> important thing is that the air is trapped and cannot move.  Moving air
[snip]

Actually, this is incorrect, R-value is literally a measure of resistance 
to heat flow through a material.  Tiny pockets of trapped air is 
usually the easiest way to achieve a high R-value material, but there is 
significant variation in heat resistance between all sorts of materials 
including straw fibers, sand and clay.  In addition to the straw 
content which will have a small effect in the combined material, how 
porous your dried mix is could be significant due to the tiny air pockets 
left when your mix dries.  Of course "significant" is relative, even if 
you by some miracle of ingredients, mixing and application managed to 
improve the cob's R-value by 50%, you still have a pathetic level of 
insulation.

Generally, Ocean is correct, do not treat cob as an insulative material in 
your designs, treat it as thermal mass.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
Phone: (800) 467-5820 |          - Natural Building Instruction -
    or: (541) 929-4089 |                  www.deatech.com