[Cob] Cob R-values

[Shannon C. Dealy]

On Fri, 5 May 2006, Tim V wrote:

[snip]
> the only factor, our walls were near 12 inches in width. We were told that
> cob had an excellent R value, around 30 i think, but not as good as a
> strawbale structure, which another project had built nearby. In other words,
> cob still better than stick frame with regular insulation. -TV

There are alot of people who over the years have handed out various 
nonsensical numbers for the R-value of cob, and these numbers have been 
passed around to the point where far to many people have heard and believe 
them.  The problem is that some people have "guessed" the value based on 
how their cob house behaves, and the effects of thermal mass have caused 
them to reach ridiculous conclusions.  Some of those numbers have been 
posted to this list over the years and on occasion I have tried to correct 
this misconception.  NOTE: some misconceptions have also come from mixing 
up metric and non-metric (USA) measurements for resistance to heat flow. 
For what follows, I am using USA measurements.

For the record, cob makes a TERRIBLE insulator, but in a moderate climate 
it's thermal mass can make the need for insulation completely irrelevant 
and give the illusion of it providing good insulation value.  For 
reference some common R-values:

    Glass wool        3.76/inch
    Most Softwoods    1.25/inch
    Most Hardwoods    0.91/inch
    Low density brick 0.20/inch
    Face brick        0.11/inch
    concrete          0.08/inch

R-value is a function of a number of factors including things like thermal 
conductivity of the material and the amount and distribution of air 
trapped in the material.  It should hopefully be obvious to most people 
that a mixture of earth with 10% straw is not going to have an R-value 
even close to that of any kind of common hardwood as it's thermal 
conductivity is much lower than that of earth.  Now cob mixes do vary a 
fair bit, but from the above, a realistic R-value number must be much less 
than 0.91/inch and greater than that of face brick - 0.11/in. (the straw 
and air pockets in cob would make it's R-value higher).  Beyond 
that I believe it is likely to be greater than the low density brick and 
usually use a value of 0.30/inch, but this is just an extremely rough 
estimate.  Based on other real world data I have seen over the years I 
believe that any number greater than 0.5 for cob is extremely unlikely.

The upshot of this is that for the above 12" walls, R-6 would be an 
extremely generous estimate of the R-value in a 12" cob wall, and I would 
view R-4 as much more realistic, but even if all of my assumptions 
above were wrong, it is still not possible for cob to have a greater 
R-value than hardwoods.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
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