[Cob] DSCN0313

[Shannon Dealy]

On Sun, 18 Jul 2010, Henry Raduazo wrote:

> (Pictures must be requested off list)
> Janet: I believe that cob has an R-value of 1 per inch so 24 inches thick 
> would be R-24. I am not sure where I read that, but R-value is beside the 
[snip]

Sorry, but the R-value of cob IS NOT anywhere near 1 per inch.

This keeps showing up over the years, but the original discussion of it as 
I recall showed this claim to be seriously flawed as the measurements did 
not adequately account for the effects of thermal mass.  Depending on your 
mix it will most likely be somewhere between R 0.35 and 0.5 (for a really 
high straw mix).

R-values of some other materials for comparison:

    Concrete:           0.08 per inch
    Low density brick:  0.20 per inch
    Stucco:             0.20 per inch
    Adobe:              0.38 per inch
    Most hard woods:    0.90 per inch
    Most soft woods:    1.25 per inch

It should be fairly obvious to anyone who has worked with these materials 
that cob cannot possibly have an R-value anywhere near as high as a 
typical hard wood, and that it is at least somewhat comparable to standard 
adobe.

Having said this, it is important to remember that the effects of thermal 
mass can radically alter the energy equation for a building to the point 
that depending on the location and time of year, the thermal mass may be 
far more important for energy savings than the R-value.  Unfortunately the 
codes have not historically recognized this fact, so it may be necessary 
to beat your building officials over the head with hard engineering 
mathematics and data in order to make use of this.

Of course, your best choice for anywhere that extreme temperatures can 
occur is a combination of insulation and thermal mass which usually would
require some kind of hybrid wall system for at least part of the 
structure such as cob-bale.

Shannon C. Dealy      |               DeaTech Research Inc.
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